Moles and Avogadro's Number

Moles and Avogadro's Number MDCAT MCQs with answers

61 past paper MCQs on Moles and Avogadro's Number, from the Fundamental Concepts of Chemistry unit of MDCAT Chemistry. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

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1.The mass of one molecule of $\mathrm{O_2}$ is: (MDCAT 2020)

  1. $6.02\times10^{23}/32$ g
  2. $32/6.02\times10^{23}$ g
  3. 32 g
  4. 0.32 g
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Correct answer: (b) $32/6.02\times10^{23}$ g

One mole of $\mathrm{O_2}$ weighs 32 g and holds $6.02\times10^{23}$ molecules, so one molecule is the quotient of the two.

2.The number of moles of $\mathrm{CO_2}$ which contain 8.0 g of oxygen is: (MDCAT 2020)

  1. 1.0
  2. 4.50
  3. 0.50
  4. 0.25
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Correct answer: (d) 0.25

One mole of $\mathrm{CO_2}$ carries 32 g of oxygen, so 8.0 g of oxygen belongs to a quarter of a mole.

3.One mole of any substance contains how many particles? (MDCAT 2020)

  1. $6.02\times10^{23}$
  2. $6.02\times10^{-23}$
  3. $6.02\times10^{32}$
  4. $3.01\times10^{23}$
Show answer

Correct answer: (a) $6.02\times10^{23}$

That count is Avogadro's number.

4.What is the number of particles in 0.25 moles of $\mathrm{CO_2}$? (MDCAT 2020)

  1. $6.022\times10^{23}$
  2. $1.505\times10^{23}$
  3. $2.00\times10^{23}$
  4. $3.01\times10^{23}$
Show answer

Correct answer: (b) $1.505\times10^{23}$

$0.25\times6.022\times10^{23}=1.505\times10^{23}$.

5.The number of moles of an element is directly proportional to the: (MDCAT 2024)

  1. mass of the element
  2. empirical formula mass
  3. molar mass of the element
  4. formula mass
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Correct answer: (a) mass of the element

$n=m/M$, and for a given element $M$ is fixed, so the number of moles follows the mass.

6.NaOH + H2SO4 → Na2SO4 + H2O. When 40g of NaOH reacts with 49g of H2SO4, the number of water molecules produced are: (NUMS MDCAT 2025)

  1. 6.02 × 1023
  2. 6.02 × 1024
  3. 1.2044 × 1023
  4. 1.204 × 1024
Show answer

Correct answer: (a) 6.02 × 1023

One mole of the acid and two of the alkali would be needed; the half mole of acid present gives one mole of water.

7.The mass of hydrogen gas needed to produce 51 g ammonia: (SIBA MDCAT 2025)

  1. 6g
  2. 9g
  3. 12g
  4. 15g
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Correct answer: (b) 9g

Fifty-one grams is three moles of ammonia, which needs four and a half moles of hydrogen, that is $9\,\mathrm{g}$.

8.Number of moles in an element is directly proportional to: (UHS MDCAT 2024)

  1. Mass of an element
  2. Empirical formula mass
  3. Molar mass of an element
  4. Formula mass
Show answer

Correct answer: (a) Mass of an element

The number of moles is the mass divided by the fixed molar mass, so it rises with the mass.

9.The type and relative amount of each isotope in an element can be found by: (UHS MDCAT 2024)

  1. R spectroscopy
  2. U. V spectroscopy
  3. Mass Spectrometry
  4. N.M.R
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Correct answer: (c) Mass Spectrometry

A mass spectrometer separates the isotopes by mass and measures how much of each there is.

10.The atomic masses of element depend upon: (UHS MDCAT 2024)

  1. Atomic number
  2. Number of electrons
  3. Number of isopotes & their abundance
  4. None of the above
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Correct answer: (c) Number of isopotes & their abundance

The atomic mass is the weighted mean over the isotopes, so it depends on which they are and how abundant.

11.No individual atom in the sample of 1 mole of Neon has a mass of 20.18 a.m.u. because it is: (UHS MDCAT 2024)

  1. Overall mass of an isobar
  2. It is a fractional mass
  3. It is molar mass of Ne
  4. Average atomic mass of Ne
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Correct answer: (d) Average atomic mass of Ne

No single atom weighs the average; $20.18$ is the mean over the isotopes of neon.

12.What is the mass of 1 mole of calcium carbonate (CaCO3)? (KMU MDCAT 2024)

  1. 50g
  2. 75g
  3. 100g
  4. 125g
Show answer

Correct answer: (c) 100g

$40+12+48$ gives a molar mass of $100\,\mathrm{g\,mol^{-1}}$.

13.How many grams of CO2 can be produced by thermally decomposing 10 moles of ZnCO3(s)? (KMU MDCAT 2024)

  1. 320
  2. 360
  3. 400
  4. 440
Show answer

Correct answer: (d) 440

Each mole of the carbonate gives a mole of carbon dioxide, so ten moles give $440\,\mathrm{g}$.

14.How many moles of NaCl are produced from 16.5g of HCl, according to the neutralization reaction? HCl(aq) + NaOH(aq) -> NaCl(aq) + H2O(l) (KMU MDCAT 2024)

  1. 0.252
  2. 0.452
  3. 0.652
  4. 0.852
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Correct answer: (b) 0.452

$16.5/36.5$ is $0.452$ mole of the acid, and each mole gives one of the salt.

15.What mass of aluminium oxide (Al2O3) is produced from 18.5g of Al metal, when it reacts completely with oxygen gas according to the following equation? 4Al(s) + 3O2(g) → 2Al2O3(s) (KMU MDCAT 2024)

  1. 30.8g
  2. 32.6g
  3. 34.9g
  4. 36.5g
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Correct answer: (c) 34.9g

$18.5/27$ is $0.685$ mole of aluminium, half of which is $0.343$ mole of oxide, that is $34.9\,\mathrm{g}$.

16.When 4 g of magnesium was heated in excess of oxygen. Calculate the theoretical yield of magnesium oxide (MgO). (KMU MDCAT 2024)

  1. 3.7g
  2. 4.2g
  3. 5.4g
  4. 6.6g
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Correct answer: (d) 6.6g

$4\,\mathrm{g}$ is $1/6$ mole of magnesium, which gives $1/6$ mole, about $6.6\,\mathrm{g}$, of the oxide.

17.Select the greatest number of particles in the following: (NUMS MDCAT 2024)

  1. 2g of Sodium (atomic mass = 23)
  2. 2g of Hydrogen gas (atomic mass of H = 1)
  3. 2g of Nitrogen gas (atomic mass of N = 14)
  4. 2g of Carbon dioxide gas (C=12, O=16)
Show answer

Correct answer: (b) 2g of Hydrogen gas (atomic mass of H = 1)

Two grams of hydrogen is a whole mole; the others are small fractions of one.

18.One a. m .u stands for (UHS MDCAT 2023)

  1. An atom of C - 12
  2. 1/12th of a carbon
  3. 1/12th of H
  4. 1 atom of all the elements
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Correct answer: (b) 1/12th of a carbon

One atomic mass unit is a twelfth of the mass of an atom of carbon-12.

19.A compound of sodium oxide has 74.2 % sodium and 25.8% of Oxygen. The empirical formula of the compound is? (UHS MDCAT 2023)

  1. NaO
  2. NaO2
  3. Na2O
  4. Na2O2
Show answer

Correct answer: (c) Na2O

Dividing by the atomic masses gives $3.23$ of sodium to $1.61$ of oxygen, a ratio of two to one.

20.For the reaction given below: N2 + 3H2 ⇌ 2NH3 How many moles N2 are required for synthesis of 4 moles of NH3? (SZABMU MDCAT 2023)

  1. 4 moles of N2
  2. 2 moles of N2
  3. 3 moles of N2
  4. 4.5 moles
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Correct answer: (b) 2 moles of N2

The equation gives two moles of ammonia for each mole of nitrogen.

21.Al reacts with O2 according to the following reaction: 4Al + 3O2 ⟶ 2Al2O3 27g of Al will react with how much of O2? (SZABMU MDCAT 2023)

  1. 8 g
  2. 16 g
  3. 24 g
  4. 32 g
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Correct answer: (c) 24 g

$27\,\mathrm{g}$ is one mole of aluminium, which needs three quarters of a mole, $24\,\mathrm{g}$, of oxygen.

22.One mole of a substance is the amount of that substance that has the same number of particles (atom, ions or molecules) as there are atoms in exactly: (SZABMU MDCAT 2023)

  1. 1.008 g of hydrogen gas (H2)
  2. 16 g of oxygen gas (O2)
  3. 12 g of carbon-12 isotopes
  4. 12 g of magnesium
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Correct answer: (c) 12 g of carbon-12 isotopes

The mole is defined by the number of atoms in exactly $12\,\mathrm{g}$ of carbon-12.

23.What is the mass of sulphur in 24.5 g of H2SO4? (NUMS MDCAT 2023)

  1. 32 g
  2. 24 g
  3. 16 g
  4. 8 g
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Correct answer: (d) 8 g

$24.5\,\mathrm{g}$ is a quarter of a mole, and a quarter of $32\,\mathrm{g}$ of sulphur is $8\,\mathrm{g}$.

24.From the equation (N2 + 3H2 → 2NH3), how many moles of NH3 are produced from 2.5 moles of N2? (NUMS MDCAT 2023)

  1. 2.5 moles
  2. 2 moles
  3. 5 moles
  4. 7.5 moles
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Correct answer: (c) 5 moles

Each mole of nitrogen gives two of ammonia, so $2.5$ moles give $5$.

25.The average weight of atoms of an element compared to the weight of one atom of _________ is called atomic weight. (UHS MDCAT 2023)

  1. Carbon
  2. Helium
  3. Hydrogen
  4. Nitrogen
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Correct answer: (a) Carbon

Atomic weights are referred to carbon-12, taken as exactly twelve units.

26.Which element is used as standard to determine atomic mass of an element? (UHS MDCAT 2023)

  1. H
  2. C
  3. P
  4. Cl
Show answer

Correct answer: (b) C

Carbon-12 is the standard against which atomic masses are measured.

27.What mass is the mass of 2 dm3 of oxygen at S.T.P? (PMC MDCAT 2022)

  1. 1.42 g
  2. 2.22 g
  3. 3.2 g
  4. 2.85 g
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Correct answer: (d) 2.85 g

$2/22.4$ mole of oxygen at $32\,\mathrm{g\,mol^{-1}}$ weighs about $2.85\,\mathrm{g}$.

28.What is the mass in grams of 1.69 moles of phosphoric acid? (PMC MDCAT 2022)

  1. 148 g
  2. 138 g
  3. 157 g
  4. 166 g
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Correct answer: (d) 166 g

$1.69\times98$ gives about $166\,\mathrm{g}$.

29.A compound of boron and hydrogen contains 18.9% hydrogen and 81.1% boron. The empirical formula of the compound is: (PMC MDCAT 2022)

  1. BeH2
  2. Be2H
  3. B4H
  4. B2H5
Show answer

Correct answer: (d) B2H5

$81.1/10.8$ to $18.9/1$ is $7.5$ to $18.9$, that is $2$ to $5$.

30.What is the percentage of C in glucose (C6H12O6)? (PMC MDCAT 2022)

  1. 41.5
  2. 38
  3. 39.9
  4. 38.9
Show answer

Correct answer: (c) 39.9

Carbon is $72$ of the $180$ units of the molar mass, that is about $40\%$.

31.One mole of ethanol and one mole of ethane have an equal: (NUMS MDCAT 2022)

  1. Mass
  2. Number of atoms
  3. Number of electrons
  4. Number of molecules
Show answer

Correct answer: (d) Number of molecules

A mole of anything holds Avogadro’s number of molecules.

32.The 1' step involved in the determination of Empirical Formula of chemical compound is: (NUMS MDCAT 2022)

  1. Finding number of gram atoms of each element
  2. Percentage composition of each element
  3. Atomic ratio of each element
  4. Multiplication of atomic ratio with whole number
Show answer

Correct answer: (b) Percentage composition of each element

The analysis gives the percentage of each element, and everything else follows from that.

33.The number of moles of CO2 which contains 16 g of oxygen is: (NUMS MDCAT 2022)

  1. 0.25
  2. 0.5
  3. 1
  4. 2
Show answer

Correct answer: (b) 0.5

Sixteen grams is one mole of oxygen atoms, and each molecule carries two, so there is half a mole.

34.Percentage composition of mass in CO2, is: (NUMS MDCAT 2022)

  1. 30.45% Carbon & 69.54% Oxygen
  2. 24.22% Carbon & 75.78% Oxygen
  3. 27.37% Carbon & 72.72% Oxygen
  4. 41.68% Carbon & 58.12% Oxygen
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Correct answer: (c) 27.37% Carbon & 72.72% Oxygen

Carbon is $12$ of the $44$ units, that is $27.3\%$, and the oxygen is the rest.

35.The relative atomic mass of copper is? (PMC Practice 2021)

  1. 63.345amu
  2. 63.455amu
  3. 63.55amu
  4. 63.456amu
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Correct answer: (c) 63.55amu

Copper has two isotopes, and the weighted mean of their masses is $63.55\,\mathrm{amu}$.

36.18g of water contains ____ atoms of hydrogen (PMC Practice 2021)

  1. 6.022 x 1023
  2. 3 x 6.022 x 1023
  3. 2 x 6.022 x 1023
  4. 4 x 6. 22 x 1023
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Correct answer: (c) 2 x 6.022 x 1023

Eighteen grams is one mole of water, and each molecule carries two hydrogen atoms.

37.According to Avogadro’s law, .899g of 1dm3 H2 and 1.4384g of 1dm3 O2 have _____________number of molecules. (PMC Practice 2021)

  1. Same
  2. Different
  3. H2 has more
  4. 02 has more
Show answer

Correct answer: (a) Same

Equal volumes of gases at the same temperature and pressure hold equal numbers of molecules.

38.Different kinds of atoms of the same element are called isotope having different __________but same ___________ properties. (PMC Practice 2021)

  1. Physical, atomic
  2. Physical, chemical
  3. Chemical, physical
  4. Chemical, atomic
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Correct answer: (b) Physical, chemical

Isotopes differ in mass and so in physical properties, but their electron arrangement, and so their chemistry, is the same.

39.Calculate the mass in grams of 8.694 moles of Ag2CO3 (PMC Practice 2021)

  1. 1417.53 g
  2. 2399.544 g
  3. 3456.78 g
  4. 1231.98 g
Show answer

Correct answer: (b) 2399.544 g

$8.694\times275.8$ gives about $2399.5\,\mathrm{g}$.

40.The empirical formula of Glucose C6H12O6 is: (PMC MDCAT 2020)

  1. C6H12O6
  2. CHO
  3. CH2O
  4. CH2O2
Show answer

Correct answer: (c) CH2O

Dividing the subscripts by six leaves $\mathrm{CH_2O}$.

41.In a vessel, 10 g N2, 10 g H2 and 10g O2 are present. Which one will have least number of atoms? (PMC MDCAT 2020)

  1. H2
  2. N2
  3. O2
  4. Both H2 and N2
Show answer

Correct answer: (c) O2

Ten grams is $5$ moles of hydrogen but only $0.31$ of oxygen, so the oxygen has fewest atoms.

42.Which two elements are isotopes? (UHS MDCAT 2019)

  1. 16X8 and 16Y8
  2. 14X8 and 15Y8
  3. 18X9 and 20Y10
  4. 12X6 and 12Y7
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Correct answer: (b) 14X8 and 15Y8

Isotopes have the same proton number and different mass numbers.

43.Solution contains 85.5 g of sucrose (C12H22O11) in 250 cm3. What is its molarity? (UHS MDCAT 2019)

  1. 1 M
  2. 0.5 M
  3. 0.25 M
  4. 2 M
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Correct answer: (a) 1 M

$85.5/342$ is $0.25$ mole in $0.25\,\mathrm{dm^3}$, that is $1\,\mathrm{M}$.

44.The average atomic mass of Boron is 10.8. It has two isotopes of masses 10 and 11 respectively. What is the percentage of isotope with the average mass of 10? (UHS MDCAT 2019)

  1. 20%
  2. 60%
  3. 80%
  4. 50%
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Correct answer: (a) 20%

$10x+11(1-x)=10.8$ gives $x=0.2$, that is $20\%$.

45.The number of moles of water in 1 kg ice are: (UHS MDCAT 2019)

  1. 50 moles
  2. 100 moles
  3. 1000 moles
  4. 55.5 moles
Show answer

Correct answer: (d) 55.5 moles

$1000/18$ is about $55.5$ moles.

46.How many moles of Calcium Carbonate are present in 1.75 kg of Calcium Carbonate (Ar of Ca = 40, Ar of C = 12, Ar of O = 16) (UHS MDCAT 2019)

  1. 1.75 mol
  2. 1750 mol
  3. 0.0175 mol
  4. 17.5 mol
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Correct answer: (d) 17.5 mol

$1750/100$ is $17.5$ moles.

47.During stoichiometric calculations, which of the following laws must be followed? (UHS MDCAT 2019)

  1. Dalton's law
  2. Avogadro's law
  3. Law of conservation of mass
  4. Law of conservation of energy
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Correct answer: (c) Law of conservation of mass

Matter is neither made nor destroyed, so the equation must balance.

48.The formula which shows the simplest whole number ratio for the atoms of different elements in a compound is: (UHS MDCAT 2018)

  1. Ionic formula
  2. Structural formula
  3. Empirical formula
  4. Molecular formula
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Correct answer: (c) Empirical formula

The simplest whole number ratio of the atoms is the empirical formula.

49.While finding the relative atomic mass, which of the following standard is used to compare the atomic mass of chlorine (35.5 amu) (UHS MDCAT 2018)

  1. Carbon – 13
  2. Neon – 20
  3. Carbon – 12
  4. Nucleon number
Show answer

Correct answer: (c) Carbon – 12

Atomic masses are referred to carbon-12, taken as exactly twelve units.

50.3.0 mole of Calcium will contain _____ g of Calcium. (UHS MDCAT 2018)

  1. 105 g
  2. 120 g
  3. 80 g
  4. 100 g
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Correct answer: (b) 120 g

$3.0\times40$ is $120\,\mathrm{g}$.

51.Determine the number of moles of O in 10.6g of Na2CO3: (UHS MDCAT 2017)

  1. 0.4 moles
  2. 0.3 moles
  3. 0.2 moles
  4. None of the given options
Show answer

Correct answer: (b) 0.3 moles

$10.6/106$ is $0.1$ mole of the carbonate, and each carries three oxygens.

52.Choose the correct option regarding number or particles associated with one mole of a substance: (UHS MDCAT 2017)

  1. 6.03 x 1023
  2. 6.01 x 10-19
  3. 6.02 x 10-23
  4. 6.02 x 1023
Show answer

Correct answer: (d) 6.02 x 1023

A mole holds $6.02\times10^{23}$ particles.

53.Calculate the grams of H2O formed when 8g of CH4 burns in excess of oxygen: (UHS MDCAT 2017)

  1. 21 grams
  2. 19 grams
  3. 18 grams
  4. 15 grams
Show answer

Correct answer: (c) 18 grams

$8\,\mathrm{g}$ is half a mole of methane, which gives one mole, $18\,\mathrm{g}$, of water.

54.A compound has an empirical formula CH2Cl and molecular formula mass as 99gmol-1, identify the compound: (UHS MDCAT 2017)

  1. C2H4Cl
  2. C4H8Cl
  3. C2H4Cl2
  4. C2H3Cl3
Show answer

Correct answer: (c) C2H4Cl2

The empirical formula weighs $49.5$, and $99/49.5$ is two, so the molecule is $\mathrm{C_2H_4Cl_2}$.

55.Among the following, which contains the same no. Of electrons & protons but different no. of neutrons: (UHS MDCAT 2017)

  1. Isboars
  2. Isotopes
  3. Isotones
  4. None of the given options
Show answer

Correct answer: (b) Isotopes

Isotopes of an element differ only in the number of neutrons.

56.What mass of NaOH is present in 0.5 mol of sodium hydroxide? (UHS MDCAT 2016)

  1. 40 gm
  2. 2.5 gm
  3. 5 gm
  4. 20 gm
Show answer

Correct answer: (d) 20 gm

$0.5\times40$ is $20\,\mathrm{g}$.

57.The number of moles of CO2 which contain 8.00 gm of oxygen is: (UHS MDCAT 2016)

  1. 0.75
  2. 1.50
  3. 0.25
  4. 1.00
Show answer

Correct answer: (c) 0.25

$8.00\,\mathrm{g}$ is half a mole of oxygen atoms, and each molecule carries two.

58.The substance for the separation of isotopes is firstly converted into the (UHS MDCAT 2016)

  1. Neutral state
  2. Free state
  3. Vapour state
  4. Charged state
Show answer

Correct answer: (c) Vapour state

Isotopes are separated in a mass spectrometer, which works on the vapour.

59.Number of neutrons in 6630Zn (UHS MDCAT 2016)

  1. 30
  2. 35
  3. 38
  4. 36
Show answer

Correct answer: (d) 36

$66-30$ leaves $36$ neutrons.

60.10.0 grams of glucose are dissolved in water to make 100 cm3 of its solution, its molarity is: (UHS MDCAT 2015)

  1. 0.55
  2. 0.1
  3. 10
  4. 1
Show answer

Correct answer: (a) 0.55

$10.0/180$ is $0.0555$ mole in $0.1\,\mathrm{dm^3}$, that is about $0.55\,\mathrm{M}$.

61.Given solution contains 16.0 g of CH3OH, 92.0 g of C2H5OH and 36 g of water. Which statement about mole fraction of the components is true? (UHS MDCAT 2015)

  1. Mole fraction of CH3OH is highest among all
  2. Mole fraction of C2H5OH and H2O is the same
  3. Mole fraction of CH3OH and C2H5OH is same components
  4. Mole fraction of H2O is the lowest among all
Show answer

Correct answer: (b) Mole fraction of C2H5OH and H2O is the same

The amounts are $0.5$, $2$ and $2$ moles, so ethanol and water share the same mole fraction.

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