Question

a) Calculate the molecular masses of the following compounds :
i) Acetic acid (CH3COOH) ii) Ethanol (C2H5OH) iii) Carbon dioxide
b) Calculate the mass of the following :
i) 0.5 mole of N2 gas(mass from mole of molecule)
ii) 0.5 mole of N atoms ( mass from mole of atom)
iii) 3.011 × 1023 number of N atoms(mass from number)
iv) 6.022 × 1023 number of N2 molecules (mass from number )
c) Calculate the number of particles in each of the following :
i) 46 g of Na atoms (number from mass)
ii) 8 g of O2 molecules ( number of molecules from mass)
iii) 0. 1 mole of carbon atoms ( number from given moles)

Answer 1

These are very simple questions provided you know the formulae.
You must memorize the atomic mass of common elements like carbon, hydrogen, oxygen, nitrogen, helium, sodium, calcium, iron and aluminium. Or you might be provided with the values in the exam.

a) For calculation of molar mass, add (individual atomic weights of the elements*number of atoms of elements in the compound).
For example, for acetic acid, Molar mass = (mass of carbon*2) + (mass of hydrogen * 4) + (atomic mass of oxygen*2) = (12*2) + (1*4) + (16*2) = 60 u

b) Mass of 1 mole of a substance in grams = Mass of one atom in u (unified mass, a unit for measuring mass of very small particles).
i) Thus mass of 1 mole of N2 gas = 2*14 = 28 grams
Mass of 0.5 mole of N2 gas = 28/2 = 14 grams

ii) Mass of 1 mole of N atoms = 14 grams
Mass of 0.5 mole of N atoms = 14/2 = 7 grams

iii) 1 mole of a substance contains 6.022 * 10^23 particles of it.
Thus 3.011 * 10^23 particles of a substance would be present in 0.5 moles of it.
Mass of 0.5 mole of N atoms is 7 grams.

iv) Mass of 6.022 number of N2 molecules = Mass of 1 mole N2 molecules = 28 grams.

c) Convert mass to moles and then to number of atoms/molecules.

i) Mass of 1 mole sodium atoms is 23 grams.
46 grams of sodium would be present in 2 moles of sodium.
2 moles of sodium contain 12.044 * 10^23 number of sodium (Na) atoms.

iii) 1 mole of carbon atoms contains 6.022 * 10^23 atoms
0.1 mole of carbon atoms contains 6.022 * 10^22 atoms.

Try to understand the steps carefully and solve the remaining questions yourself. Please ask if you have any doubts.

Answer 2

acetic acid=12+3+12+32+1=48
ethanol=24+5+16+1=46
carbon dioxide=12+16+16=44

i mole n2=28
0.5 mole n2=14
0.5 mole n=7

3.011*1023 n atom=7
6.022 × 1023 number of N2 molecules =34

46 g Na= 2 mole
8g o2=i/2 mole= 3.022*10^23