Define formula unit mass of a compound. Calculate the formula unit mass of:
$(a)$ $Na_{2}CO_{3} \cdot 10H_{2}O$
$(b)$ $CuSO_{4}$
(Given atomic mass of $Na = 23 \text{ u}, C = 12 \text{ u}, O = 16 \text{ u}, Cu = 63.5 \text{ u}, S = 32 \text{ u}, H = 1 \text{ u}$)

(N/A) The formula unit mass of a substance is the sum of the atomic masses of all atoms in a formula unit of a compound. It is calculated in the same manner as molecular mass,with the distinction that the term 'formula unit mass' is used for substances whose constituent particles are ions,such as $NaCl$.
$(a)$ $Na_{2}CO_{3} \cdot 10H_{2}O = (2 \times \text{Mass of } Na) + (1 \times \text{Mass of } C) + (3 \times \text{Mass of } O) + 10 \times (2 \times \text{Mass of } H + 1 \times \text{Mass of } O)$
$= (2 \times 23) + (1 \times 12) + (3 \times 16) + 10 \times (2 \times 1 + 16)$
$= 46 + 12 + 48 + 10 \times (18)$
$= 106 + 180 = 286 \text{ u}$
$(b)$ $CuSO_{4} = (1 \times \text{Mass of } Cu) + (1 \times \text{Mass of } S) + (4 \times \text{Mass of } O)$
$= 63.5 + 32 + (4 \times 16)$
$= 63.5 + 32 + 64 = 159.5 \text{ u}$