# Chemical Engineering Questions and Answers – Mole Fraction and Mass(Weight) Fraction

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This set of Basic Chemical Engineering Multiple Choice Questions & Answers (MCQs) focuses on “Mole Fraction and Mass(Weight) Fraction”.

1. What is the mole fraction of sodium with moles n in a solution with moles of solvent N?
a) N/n
b) n/N
c) n/(n + N)
d) N/(n + N)

Explanation: Mole fraction is the ratio of moles of solvent and the total number of moles of solution, => mole fraction = n/(n + N).

2. What is the unit of mole fraction?
a) Kg/m3
b) N/m3
c) m-2
d) None of the mentioned

Explanation: Since the mole fraction is ratio of the moles of solute and the moles of solution, it has no unit.

3. If the mole fraction of 4.5 moles of solute is 0.5, what is the number of moles of solvent?
a) 9
b) 4.5
c) 1.5
d) 3

Explanation: 0.5 = 4.5/(4.5 + n), => n = 4.5.
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4. What is the mass fraction of 4 moles of Sodium in 8 moles of Hydrogen chloride?
a) 0.24
b) 0.48
c) 0.56
d) 0.72

Explanation: Mass of sodium = 4*23 = 92 g, mass of hydrogen chloride = 8*36.5 = 292 g, => mass fraction of sodium = 92/(92 + 292) = 0.24.

5. If the mole fraction of sodium is 0.25 in 9 moles of liquid carbon dioxide, what is the mass fraction of sodium?
a) 0.10
b) 0.15
c) 0.20
d) 0.25

Explanation: 0.25 = n/(n + 9), => n = 3, => mass of sodium = 3*23/(3*23 + 9*44) = 0.15.

6. A solution has 25% w/w solution of Sodium Chloride in 10 grams of water what is the mole fraction of Sodium Chloride?
a) 0.05
b) 0.06
c) 0.07
d) 0.08

Explanation: Mass of sodium chloride = 10*25/100 = 2.5 grams, => moles of sodium chloride = 2.5/58.5 = 0.0427, moles of water = 10/18 = 0.555, => mole fraction = 0.0427/(0.0427 + 0.555) = 0.07.

7. If the mole fraction of SO2 in H2SO4 is 0.2, what is the ratio of mass of sulfur and oxygen in the solution?
a) 0.166
b) 6
c) 1.8
d) 0.555

Explanation: Let moles of SO2 and H2SO4 be x and y respectively, => x/(x + y) = 0.2, => y/x = 4, => ratio of mass of sulfur and oxygen in solution = (x*32 + y*32)/ (x*2(16) + y*4(16)) = 0.555.

8. A mixture of gases SO2, O2 and 4 g of N2 and total mass of mixture is 10 g, what is the sum of mole fraction of SO2 and O2?
a) 0.25
b) 0.5
c) 0.75
d) 1

Explanation: Sum of the mole fraction of all gases = 1, => sum of mole fraction of SO2 and O2 = 1 – 4/10 = 0.75.

9. A neutral gas that is made of nitrogen and hydrogen only contains 7 g of nitrogen is in a mixture of with total mass 17 g, what is the mass fraction of the gas?
a) 0.5
b) 0.25
c) 0.45
d) 0.8

Explanation: The neutral gas that is made of nitrogen and hydrogen is NH3, => moles of nitrogen = moles of NH3 = 7/14 = 0.5, => mass of hydrogen = 1.5*1 = 1.5 g => total mass of NH3 = 7 + 1.5 = 8.5 g, => mass fraction = 8.5/17 = 0.5.

10. A mixture contains 23 g of ethanol and 36 g of water, some acetic acid is added to the mixture and its mole fraction becomes 0.5, how much acetic acid was added?
a) 25 g
b) 75 g
c) 100 g
d) 150 g

Explanation: Let m grams of acetic acid was added, => 0.5 = (m/60)/{(23/46) + (36/18) + (m/60)}, => m = 150 g.

11. A mixture contains 9 gram each of H2O and NaCl, what is the mole fraction of NaCl?
a) 0.15
b) 0.23
c) 0.45
d) 0.64

Explanation: Mole fraction of NaCl = (9/58.5)/[9/58.5 + 9/18] = 0.23.

12. A mixture contains 10 grams each of CH4, H2O, and SO2, what is the mole fraction of CH4?
a) 0.14
b) 0.31
c) 0.46
d) 0.67

Explanation: Mole fraction of CH4 = (10/16)/[(10/16) + (10/18) + (10/64)] = 0.46.

13. A mixture contains 80% of HCl and 20% H2O by weight, what is the mole fraction of HCl?
a) 0.15
b) 0.29
c) 0.45
d) 0.66

Explanation: Mole fraction of HCl = (80/36.5)/[(80/36.5) + (20/18)] = 0.66.

14. A mixture contains 80% of NaOH and 20% H2O by weight, what is the mole fraction of NaOH?
a) 0.24
b) 0.42
c) 0.64
d) 0.78

Explanation: Mole fraction of HCl = (80/40)[(80/40) + (20/18)] = 0.64.

15. A compound contains 40% of Sulphur and 60% of Oxygen, what is the empirical formula of the compound?
a) SO2
b) SO3
c) SO4
d) None of the mentioned

Explanation: Mole fraction of Sulphur = (40/32)[(40/32) + (60/16)] = 0.25, => Mole fraction of oxygen = 0.75, => Empirical formula of compound is SO3.

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