Question

A compound with molar mass 159 was found to contain 39.62% copper and 20.13% sulfur. Find the molecular formula for the compound. (Cu=63, S=32, O=16).

Answer 1

$\%$b copper + $\%$ sulphur $=39.62+20.13$

$=59.75$

This is less than 100% Hence compound contains adequate quantity of oxygen so that total percentage of elements is 100%

Hence of oxygen = $100-59.75=40.25\%$

Element	% of element	At. mass	Atomic ratio	Simplest ratio
Cu	$39.62$	$63$	$\frac{39.62}{63}=0.63$	$\frac{0.63}{0.63}=1$
S	$20.13$	$32$	$\frac{20.13}{32}=0.63$	$\frac{0.63}{0.63}=1$
O	$40.25$	$16$	$\frac{40.25}{16}=2.51$	$\frac{2.51}{0.63}=4$

Hence empirical formula is $CuS{O}_4$

Molecular weight = $63+32+16\times 4$

= $159$