Question

calculate the percentage by mass of the named elements in the following compounds:

(a)Al in KAl₂(SO₄)₃.12H₂O

(b)Na in Na₂SO₄.10H₂O

Answer 1

1) Mass % = $\frac{massofthecomponent}{totalmassofthesubs\tan ce}\times 100$
atomic mass of Al = 27 u
​​atomic mass of K = 39 u
atomic mass of S = 32 u
atomic mass of O = 16 u
atomic mass of H = 1 u
So total mass of KAl₂(SO₄)₃.12H₂O =
$$\begin{gathered} [39+2\times 27+3\times 32+12\times 16+12\times 18] \\ =597g \end{gathered}$$

mass % of Al = $\frac{54}{597}\times 100$
= 9.045 %

2) atomic mass of​ Na = 23 u
​atomic mass of S = 32 u
atomic mass of O= 16 u
atomic mass of H = 1 u
Total mass of Na₂SO₄. 10H₂O
= $[2\times 23+32+4\times 16+10\times 18]$
= 322 g
mass % of Na = $\frac{46}{322}\times 100$
= 14.286 %