## Calculate the molar mass of the following substances.

(a) Ethyne, C
_{
2
}
H
_{
2
}

(b) Sulphur molecule, S
_{
8
}

(c) Phosphorus molecule, P
_{
4
}
(Atomic mass of phosphorus = 31)

(d) Hydrochloric acid, HCl

(e) Nitric acid, HNO
_{
3
}

**
Answer
**

Molar mass of C = 12g

Molar mass of H = 1g

Molar mass of S = 32g

Molar mass of P = 31g

Molar mass of Cl = 35.5g

Molar mass of N = 14g

Molar mass of O = 16g

####
(a)
**
Ethyne, C
**
_{
2
}
**
H
**
_{
2
}

**
Molar mass of C
**
_{
2
}
**
H
**
_{
2
}
= 2 x Molar mass of C + 2 x Molar mass of H

= 2x 12g + 2 x 1g

= 24g + 2g

=
**
26g
**

####
(b)
**
Sulphur molecule, S
**_{
8
}

_{ 8 }

**
Molar mass of S
**
_{
8
}
= 8 x Molar mass of S

= 8x 32g

=
**
256g
**

####
(c)
**
Phosphorus molecule, P
**
_{
4
}

**
Molar mass of P
**
_{
4
}
= 4 x Molar mass of P

= 4 x 31g

=
**
124g
**

####
(d)
**
Hydrochloric acid, HCl
**

**
Molar mass of HCl
**
= 1 x Molar mass of H + 1 x Molar mass of Cl

= 1x 1g + 1 x 35.5g

= 1g + 35.5g

=
**
36.5g
**

####
(e)
**
Nitric acid, HNO
**
_{
3
}

**
Molar mass of HNO
**
_{
3
}
= 1 x Molar mass of H + 1 x Molar mass of N + 3 x Molar mass of O

=1x 1g + 1 x 14g + 3 x 16g

= 1g + 14g + 48g

=
**
63g
**