The molecules of an element are constituted by the same type of atoms. Molecules of many elements, such as argon (Ar), helium (He), and others, have only one atom in their molecules. However, this is not the case with the majority of nonmetals. A molecule of oxygen, for example, is a diatomic molecule because it contains two oxygen atoms. The sum of the atomic mass of each atom in a molecule is its molecular mass.
| Definition:The sum of the atomic masses of all the atoms in a molecule of a substance is the molecular mass. As a result, it is a molecule’s relative mass represented in atomic mass units (u). |
What is molecular mass and how to find molecular mass Questions with Solutions
Q-1: Give the general steps for determining molecular mass of the molecule.
Answer: Following the given components, we may get the molecule’s molecular mass:
- Determine the compound or molecule’s formula.
- Determine the number of atoms in each element of the compound or molecule using the formula.
- Multiply the atomic weight of each element by the number of atoms of that element.
- Add together all of the values.
Q-2: Write the chemical formula and calculate the molecular mass of the following.
- Sodium sulphite
- Quick lime
- Sulphur molecule
Answer:
a) Sodium sulphite is represented by the chemical formula Na2SO3.
The atomic mass of Na, S and O atoms are 23 u, 32 u and 16 u respectively.
Molecular mass of HBr = 2(23)+32+3(16) u= 126 u
b) Quick lime is represented by the chemical formula CaO.
The atomic mass of Ca and O atoms are 40 u and 16 u respectively.
Molecular mass of quicklime = (40+16) u = 56 u
c) Sulphur molecule is represented by the chemical formula S8
The atomic mass of S atom is 32 u
Molecular mass of sulphur molecule = 32×8= 256 u
Q-3: A compound contains 34.8% oxygen, 52.2% carbon and 13% hydrogen. The empirical formula mass (in u) is
- 46
- 50
- 42
- 45
Answer: a) 46
Explanation:
Since we are having mass per cent, it is convenient to use 100 g of the compound as the starting material.
| Element | Gram weight | Atomic mass( in u) | Moles(n)
n= g weight/ atomic mass |
Simplest ratio |
|---|---|---|---|---|
| O | 34.8 g | 16 | 2.175 | 1 |
| C | 52.2 g | 12 | 4.35 | 2 |
| H | 13 g | 1 | 13 | 6 |
The empirical formula will be C2H6O.
Empirical formula mass = 2(12)+6(1)+16 = 46 u
Note: Divide each of the mole values obtained by the smallest number among them for the simplest ratio.
Q-4: Match the chemical formulas in Column I with their molecular masses in Column II.
| Column I | Column II |
|---|---|
|
a) 74 u |
|
b) 180 u |
|
c) 60 u |
|
d) 101 u |
|
e) 81.4 u |
Answer: 1-b), 2-c), 3-d),4-a), 5-e)
Q-5: What do you mean by formula mass?
Answer: The component units of some compounds, such as potassium chloride, are not distinct molecules. Positive (potassium ion) and negative entities (chloride ion) are arranged in a three-dimensional structure in such compounds. The phrase formula mass is applied to these substances. It is determined similarly to molecular mass.
Q-6: The percentage by weight of aluminium in aluminium nitride is
- 42.4%
- 27.27%
- 65.85%
- 17.03%
Answer: c) 65.85%
Explanation:
Molecular mass of aluminium nitride, AlN= 27 u +14 u= 41 u
Q-7: 1.89 g of the metal ‘X’ is present in 4g of metallic chloride. Find the empirical formula and empirical formula mass of the metallic chloride.(Atomic mass of X=64 u)
Answer: Mass of metallic chloride = 4g
Mass of metal ‘X’ =1.89g
Mass of chloride = 4-1.89 = 2.11 g
Empirical formula gives the simple whole number ratio of the atoms of an element present in the compound.
| Element | Mass of the element | Atomic mass( in u) | Number of moles | Simplest ratio |
|---|---|---|---|---|
| X | 1.89 | 64 | 1.89/64=0.03 | 0.03/0.03=1 |
| Cl | 2.11 | 35.5 | 2.11/35.5=0.06 | 0.06/0.03=2 |
Empirical formula is XCl2
Empirical formula mass = 1(X)+2(Cl)= 64 + 71= 135 u
Q-8: CaSO4.xH2O (Hydrated copper sulphate) contains 21% of water of crystallisation. The value of ‘x’ is
a) 1
b) 2
c) 3
d) 4
Answer: b) 2
Explanation: Let the molecular mass of CaSO4.xH2O be W
Mass of X molecules of water = 21% of W
Mass of CaSO4 present in it = (100-21)% of W= 79% of W
Molecular weight of CaSO4 = 1(Ca)+1(S)+4(O)= 40 + 32+4(16)= 136 u
This implies, 136 = 79% of W =(79/100)×W
W= (100×136)/79
Molecular mass of H2O(water)= 2(H)+1(O) = 2(1)+1(16)= 18 u
Number of molecules of water,X = Mass of X molecules/Molecular weight of water
Therefore, x= 2
Q-9: How many atoms of cobalt are in a 0.59 mol of Co(C2H3O2)3?
Answer:
Given: Number of moles of Co(C2H3O2)3 = 0.59 mol
Number of cobalt atoms in Co(C2H3O2)3 = n× NA
Here,”n” is the number of cobalt atoms in a compound and NA is the Avogadro’s number.
Number of cobalt atoms in Co(C2H3O2)3 = 1× 6.022×1023 = 3.55×1023 atoms
Q-10: For 0.39 mol of bauxite, calculate
- Molecular mass of the bauxite
- Amount of Al in grams in bauxite
Answer:
1. The chemical formula of bauxite is Al2O3.2H2O
Molecular mass of Al2O3.2H2O = 2(Al)+3(O)+2(H2O)= 2(27)+3(16)+2(18)= 138 u
2. Let the weight of Al in “g” be y.
Moles of Al in Al2O3.2H2O = 2×0.39= 0.78 mol
Mole of Al = Amount of Al/ Atomic mass of Al
0.78= (y g)/(27gmol-1)
y= 21.06 g
Hence amount of Al in bauxite is 21.06 g
Q-11: Calculate the molecular mass of magnesite to three significant figures using the table of atomic weights.
Answer: The chemical formula of magnesite is MgCO3
Atomic weight of Mg = 24.305 u
Atomic weight of C = 12.011 u
Atomic weight of O = 15.999 u
Molecular mass of Magnesite = 1(Mg)+1(C)+3(O)= 1(24.305)+1(12.011)+3(15.999)= 84.313 u
Molecular mass of Magnesite upto three significant figures = 84.3 u
Note: All non zero digits are always significant.
Q-12: What is molar mass? How is it different from molecular mass?
Answer: Molar mass refers to the mass of one mole of a substance.
The difference between the molar mass and molecular mass is given below:
| Molar mass | Molecular mass |
|---|---|
| 1) It is defined for a mole | It is defined for a molecule |
| 2) It is basically the mass of the Avogadro number of atoms/molecules or compounds. | It is basically the sum of the atomic masses of all the atoms in a substance’s molecule. |
| 3) Mass of 1 mol of nitrogen is 14.0067 g. Therefore molar mass is 14.0067 g/mol | Mg(OH)2 has a molecular mass of 58 atomic mass unit. |
Q-13: What is equivalent weight? How is it related to molecular weight?
Answer: An equivalent weight, also known as gram equivalent, is the mass of one equivalent, that is, the mass of one substance that will combine with or displace a fixed quantity of another substance.
It can be calculated upon dividing the molecular weight of the substance by its valency.
Q-14: Calculate the equivalent weight of silver nitrate.
Answer: In silver nitrate(AgNO3) , the cation is Ag+ and the anion is NO3–
Molecular mass of silver nitrate = 1(Ag)+1(N)+3(O)=1(108)+1(14)+3(16)= 170 u
Equivalent weight = Molecular mass of silver nitrate/Valency
Valency is the magnitude of total charge present on the cation or anion. It is equal to 1
Hence, Equivalent weight = Molecular mass of silver nitrate= 170 u
Q-15: Do all molecules have molecular mass?
Answer: Yes, because matter is defined as anything that has mass and occupies space, it should come as no surprise that atoms and molecules have mass. Individual atoms and molecules, on the other hand, are very small, and their masses are also very small and can vary from each other.
Practise Questions on What is molecular mass and how to find molecular mass Questions with Solutions
Q-1: What is the formula of hydrogen selenide? Calculate the mass percentage of selenide in the compound.
Q-2: The unit of molecular mass is
- Dalton
- Unified atomic mass
- Atomic mass unit
- g/mol
Q-3: What is the number of moles of anion in 2.868 kg of aluminium sulphate?
Q-4: Which fertiliser among urea and ammonium phosphate is better?
Q-5: What is atomicity of a molecule? How can you obtain atomicity from the molecular mass of a homonuclear molecule?
Click the PDF to check the answers for Practice Questions.
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