# Class 11 Chemistry MCQ – Atomic and Molecular Masses

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This set of Class 11 Chemistry Chapter 1 Multiple Choice Questions & Answers (MCQs) focuses on “Atomic and Molecular Masses”.

1. As, per the current system, carbon-12 has been taken as the standard for measuring atomic masses.
a) True
b) False

Explanation: 1 amu is defined as a mass that is almost equal to one-twelfth of the mass of one carbon – 12 atom. It is a unit to measure atomic and molecular mass. The carbon atom is taken relatively here. And also 1 amu = 1.66056×10-24 g.

2. What is the mass of hydrogen in terms of amu?
a) 1.0020 amu
b) 1.0180 amu
c) 1.0070 amu
d) 1.0080 amu

Explanation: The mass of a hydrogen atom is 1.6736×10-24 g. When converted in terms of amu, 1.6736×10-24 g should be divided by 1.66056×10-24 g. 1.6736×10-24 g/1.66056×10-24 g = 1.0078 amu = 1.008 amu. This the process to measure any atomic mass in amu.

3. What is the abbreviation of amu?
a) Atomic matter unit
b) Atomic mass unified
c) Atomic mass unit
d) At mass unity

Explanation: The Atomic mass unit is a standard unit of mass that measures mass on an atomic or molecular scale. One unit of it is practically equal to 1.66056×10-24 g. 1 amu is defined as a mass that is almost equal to one-twelfth of the mass of one carbon – 12 atom.
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4. Nowadays, “amu” is replaced by ____
a) u
b) g
c) kg
d) am

Explanation: Presently, “amu” has been replaced by “u”. Atomic mass unit is “amu”, but now as it has been changed to “u”, & now known as unified mass. One unified atomic mass unit is the mass of one nucleon and is also equal to 1 g/mol.

5. Calculate the average atomic mass of nitrogen present in the atmosphere?

ISOTOPE ABUNDANCE ATOMIC MASS
14N 99.69% 14.0031 amu
15N 0.39% 15.0001 amu

a) 14.007 amu
b) 15.001 amu
c) 14.000 amu
d) 14.0031 amu

Explanation: The formula for finding the average atomic mass of an element is given by Avg. Atomic Mass = ∑Abundance of isotope x Mass of isotope. So for nitrogen, avg atomic mass = 99.69 x 14.0031 + 0.39% x 15.0001 = 14.007.

6. A sample of carbon that contains 70% carbon-12 and 30% carbon-14. What do you think is the average atomic mass of this sample?
a) 14.5
b) 14.14
c) 14
d) 12

Explanation: Given that, carbon-12 is of 70% and carbon-14 is of 30%. In order to find avg atomic mass, you should add the products of multiplications 12 with 70/100 and 14 with 30/100. 12 x 0. 7 = 9.8; 14 x 0.3 = 4.2. Now adding them 9.8 + 4.2 = 14.

7. ______ is the sum of atomic masses of the elements present in a molecule.
a) Average atomic mass
b) Atomic mass
c) Gram formula mass
d) Molecular mass

Explanation: Molecular mass is the sum of atomic masses of the elements present in a molecule. Gram formula mass is the amount of a compound with the exact mass in grams as the formula mass in amu. The standard unit of mass that measures mass on the molecular scale or an atomic scale is “amu”.

8. What’s the molecular mass of carbon dioxide?
a) 43
b) 28
c) 44
d) 40

Explanation: The individual mass of carbon is 12 amu and the individual mass of oxygen is 16 amu. The formula for carbon dioxide is CO2. Thus one carbon and two oxygens. 12(carbon M.wt) + 2 x 16(oxygen M.wts) = 12 + 32 = 44. Therefore the molecular mass of carbon dioxide is 44.

9. What’s the formula mass of NaCl?
a) 23 u
b) 35.5 u
c) 58 u
d) 58.5 u

Explanation: As Cl in NaCl can’t exist in the solid state, we can only calculate it’s formula mass. the individual mass of sodium is 23 u, whereas that of chlorine is 35.5 u. Together formula mass f Sodium chloride is the sum of individual masses, 23 + 35.5 = 58.5 u.

10. Calculate the molecular mass of sucrose(C12H22O11) molecule?
a) 342 amu
b) 343 amu
c) 341 amu
d) 340 amu

Explanation: The individual mass of carbon is 12 amu, the individual mass of hydrogen is 1 amu & the individual mass of oxygen is 16 amu. But sucrose has 12 carbons, 22 hydrogens and 11 oxygens, that is 12 x 12 + 22 x 1 + 11 x 16 = 342 amu.

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