Molecular Mass Definition: Definition of Molecular Mass, Relationship between amu & Gram, Gram molecular mass, Practice Problems & FAQs

Have you ever played see-saw with your friends?

Well, I believe most of us have played this during our childhood.

Imagine you and your best friend went to a park and decided to play see-saw to revive some childhood memories.

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I guess you must be feeling nostalgic. The mechanism of the see saw is very simple to understand, the heavier mass will remain at the bottom and the lighter mass will go up. The judgment to decide who will go up and who will go down becomes a little easier for bigger things.

Now imagine if we play a see saw with water molecule and ammonia molecule.

How will you decide which one is heavier?

Obviously, first we have to calculate their respective masses. Compounds are made up of two or more than 2 elements. Hence by combining the atomic masses of their individual atoms with respect to their stoichiometry will give us the molecular masses of the desired compound.

Table of Content

Molecular mass
Relationship between Atomic mass unit & gram
Gram Molecular mass (G.M.M)
Practice Problems
Frequently Asked Questions (FAQs)

Definition of Molecular mass:

The mass of a given molecule of a compound is referred to as molecular mass. In other words, it can also be defined as the sum of the mass of all the atoms comprising the respective molecule is the molecular mass of the given molecule. It is generally measured in ‘amu’.

Example. Calculate the molecular mass of formic acid(HCOOH)

Solution:

Molecular mass of HCOOH=

$= (2 \times a t o m i c m a s s o f H a t o m) + (1 \times a t o m i c m a s s o f C a t o m) + (2 \times a t o m i c m a s s o f O a t o m)$

$= (2 \times 1 a m u) + (1 \times 12 a m u) + (2 \times 16 a m u) = 46 a m u$

Molecule	Molecular mass
N2	28 amu
NH3	17 amu
NaOH	40 amu

Example. Calculate the molecular mass of sodium chloride (NaCl)

Solution:

Molecular mass of NaCl=

$= (1 \times a t o m i c m a s s o f N a a t o m) + (1 \times a t o m i c m a s s o f C l a t o m)$

$= (1 \times 23 a m u) + (1 \times 35.5 a m u) = 58.5 a m u$

Relationship between Atomic mass unit & gram:

1 amu = $\frac{1}{12} \times m a s s o f o n e C^{12} a t o m$

Atomic mass unit (amu) is equal 112 of the mass of one atom of C12 isotope.

$1 a m u = \frac{1}{12} \times 12 \times m a s s o f o n e n u c l e o n = m a s s o f 1 n u c l e o n \approx 1.66 \times 10^{- 24} g$

Gram Molecular mass (G.M.M):

Mass of 1 molecule expressed in grams or mass of 6.0221023 molecules.

Example. . Calculate gram molecular mass of water (H2O)

Solution:

Molecular mass of H2O= (2 atomic mass of H atom) + (1 atomic mass of O atom)

$= (2 \times 1 a m u) + (1 \times 16 a m u) = 18 a m u$

Gram molecular mass of H2O= (2 GAM of H atom) + (1 GAM of O atom)

= (2 1 g) + (1 16 g) = 18 g

Molecule	Gram Molecular mass
O2	32 g
H2SO4	98 g
HCl	36.5 g

Practice Problems:

Q 1. Which of the following represents the gram molecular mass of Formaldehyde?

30 g
18 g
3.6 g
56 g

Answer: (A)

Solution: Molecular mass of HCHO=

(2 atomic mass of H atom) + (1 atomic mass of C atom) + (1 atomic mass of O atom)

$= (2 \times 1 a m u) + (1 \times 12 a m u) + (1 \times 16 a m u) = 30 a m u$

Gram molecular mass of HCHO=

(2 GAM of H atom) + (1 GAM of C atom) + (1 GAM of O atom)

$= (2 \times 1 g) + (1 \times 12 g) + (1 \times 16 g) = 30 g$

Q 2. What will be the molar mass of oxalic acid?

14 g/mol
34 g/mol
90 g/mol
54 g/mol

Answer: (C)

Solution: The molar mass of oxalic acid C2H2O4 can be determined by adding the atomic masses of 4 O , 2 H and 1 C. According to the periodic table, oxygen has an atomic mass of 16.00 g/mol, hydrogen has an atomic mass of 1 g/mol and carbon has an atomic mass of 12.011 g/mol. So,

Mass of $C_{2} H_{2} O_{4} = 4 O \times 16.00 g / m o l + 2 C \times 12.011 g / m o l + 2 g / m o l = 90 g / m o l .$

Q 3. Find the total mass in grams of 3 moles of NaOH?

317.5 g
120 g
234.34 g
342.76 g

Answer: (B)

Solution: number of moles = $\frac{w e i g h t o f s u b s t a n c e (g)}{M o l a r m a s s}$

Molar mass of NaOH=40 g/mol

$W e i g h t o f s u b s t a n c e = n u m b e r o f m o l e s \times M o l a r m a s s$

$W e i g h t o f s u b s t a n c e = (3 \times 40) g = 120 g$

Q 4. Which of the following is the correct unit molecular mass?

g/litre
g/mol
amu
None of the above

Answer: (C)

Solution:

The total mass of the atoms required to form a molecule per mole, expressed in grams, is what makes up the molar mass, also known as the molecular weight. Molar mass is measured in grams/mole.

Frequently Asked Questions-FAQs:

1. Can relative atomic mass be defined in respect of other elements than carbon?
Answer: yes, mathematically we define relative atomic mass with respect to any other element but according to the scientific community carbon is taken as standard.

Standard definition; Relative atomic mass = Mass of one atom of any element112 mass of one C12 atom

In respect of oxygen; Relative atomic mass = Mass of one atom of any element116 mass of one O16 atom

2. What is the difference between atomic weight and atomic mass?
Answer: Atomic mass is generally used for single isotopes but atomic weight is generally the weighted average of isotopic abundance in nature. Atomic mass is always a whole number because it has a relation with the number of protons and neutrons in the nucleus.

3. Can the relative atomic mass of any element be a fraction?
Answer: No, relative atomic mass can never be fractions because it is simply the total number of protons and neutrons. Protons and neutrons are never present in fractions.

4. If relative atomic mass can not be a fraction why somewhere is fractional atomic mass or molar mass mentioned, e.g- Cl35.5
Answer: Cl35.5 written because of its isotopes, Cl present in nature in different isotopes in different natural abundance (%). So, due to their isotopes, they have fractional mass. We can call it weighted average atomic mass.