# Calculating percentage composition after drying

A partially dried clay mineral contains 8% water. The original sample contains nearly 12% water and 45% silica. The percentage of silica in the partially dried sample is nearly

1. 50%
2. 49%
3. 55%
4. 47%

I know the formulas for finding percentage composition of elements in a given compound. But how am I to use them here? Please help.

## 2 Answers

Is not a matter of percentage of elements in a compound. Take a initial mass of $100~\mathrm{g}$. Initially you have $12~\mathrm{g}$ of water, $45~\mathrm{g}$ of silica and a rest of $43~\mathrm{g}$. If the final sample has $8\%$ of water, you still have $45~\mathrm{g}$ of silica and $43~\mathrm{g}$ of the rest. So by a simple calculation:

$$x~\mathrm{g}\text{(water)}\cdot 100 / (43 + 45 + x) = 8$$

From here you obtain the whole mass and the final $\%$ of silica: $47\%$ (answer d).

The answer is 47%, obtained as $\dfrac{8}{8 + 88} \times 100\% =47$

• Welcome to Chemistry.SE! For formatting help visit the help center and for more information about this page, take the tour. Your answer might contain the correct answer, but it is more than cryptic, please edit it to include some explanation. Thank you! – Martin - マーチン Jul 8 '14 at 8:10