Question

Calculate the formula unit masses of ZnO, $N{a_2}O$, ${K_2}C{O_3}$, given atomic masses of $Zn{\text{ }} = {\text{ }}65u$,$Na{\text{ }} = {\text{ }}23u$, $K{\text{ }} = {\text{ }}39u$, $C{\text{ }} = {\text{ }}12u$, $O{\text{ }} = {\text{ }}16u$

Answer 1

When calculating the formula unit masses of any compound, we must remember that the total mass of the compound is equivalent to the sum of the atomic masses of the constituent elements multiplied by the number of atoms present.

Complete Step-by-Step Answer:
To put the above relation in a mathematical format, we can say that the formula unit mass for a compound with the molecular formula ${A_x}{B_y}{C_z}$ can be represented as:
Unit mass of (${A_x}{B_y}{C_z}$) = (x) (atomic mass of A) + (y) (atomic mass of B) + (z) (atomic mass of C)
Hence, applying this formula to the given compounds we get,
Unit mass of (ZnO) = (1) (atomic mass of Zn) + (1) (atomic mass of O)

{\; = {\text{ }}\left( {65} \right){\text{ }} + {\text{ }}\left( {16} \right)} \\\ {\;\;\;\;\; = {\text{ }}81u} \\\ \; \end{array}$$ Unit mass of ($$N{a_2}O$$) = (2) (atomic mass of Na) + (1) (atomic mass of O) $$\begin{array}{*{20}{l}} {\;\;\;\;\; = {\text{ }}\left( 2 \right){\text{ }}\left( {23} \right){\text{ }} + {\text{ }}\left( {16} \right)} \\\ {\;\;\;\;\; = {\text{ }}46{\text{ }} + {\text{ }}16} \\\ {\;\;\;\;\; = {\text{ }}62u} \end{array}$$ Unit mass of ($${K_2}C{O_3}$$) = (2) (atomic mass of K) + (1) (atomic mass of C)+(1) (atomic mass of O) $$\begin{array}{*{20}{l}} { = {\text{ }}\left( 2 \right){\text{ }}\left( {39} \right){\text{ }} + {\text{ }}\left( 1 \right){\text{ }}\left( {12} \right){\text{ }} + {\text{ }}\left( 3 \right){\text{ }}\left( {16} \right)} \\\ {\;\;\;\;\; = {\text{ }}78{\text{ }} + {\text{ }}12{\text{ }} + {\text{ }}48} \\\ {\;\;\;\;\; = {\text{ }}138u} \end{array}$$ **Hence, the unit masses of the given compounds has been calculated.** **Note:** Molecular mass or unit mass of a substance is a characteristic property of a compound and always remains constant. The only time the molecular weight of a compound may vary is when another isotope of a constituent element is used.