Question

Calculate the formula unit mass of:

1. $\mathrm{ZnO}$ (it) ${\mathrm{Na}}_2\mathrm{O}$ (iii) ${\mathrm{K}}_2{\mathrm{CO}}_3$

Given : Atomic masses of Zn=65 u, Na= 23u, K = 39 u, C = 12 u and O = 16 u.

Answer 1

• Formula unit mass is defined as the sum of atomic mass of all the atoms in formula unit of compound.
• It is used for those substances whose constituent element is ions.

Explanation of Part 1:

The formula unit mass of $\mathrm{ZnO}$=$(1\times atomicmassofZn)+(1\times atomicmassofO)$

$$\begin{gathered} =(1\times 65)+(1\times 16) \\ =81u \end{gathered}$$

Explanation of Part 2:

The formula unit mass of ${\mathrm{Na}}_2\mathrm{O}$=$$\begin{gathered} (2\times atomicmassofNa)+(1\times atomicmassofoxygen) \\ =(2\times 23)+(1\times 16) \\ =62u \end{gathered}$$

Explanation of Part 3:

The formula unit mass of ${\mathrm{K}}_2{\mathrm{CO}}_3$=$$\begin{gathered} (2\times atomicmassofpotassium)+(1\times atomicmassofcarbon)+(3\times atomicmassofoxygen) \\ =(2\times 39)+(1\times 12)+(3\times 16) \\ =138u \end{gathered}$$