Question

Complete combustion of $0.858g$ of compound $X$ gives $2.63g$ of $C{O}_2$ and $1.28g$ of $H_{2}O$. The lowest molecular weight which $X$ can have, is:

• A. $43g$
• B. $86g$
• C. $129g$
• D. $172g$

Answer 1

The correct option is A $43g$

Given
molar mass of $C{O}_2$ ​=2.630g or since the molar mass of $C{O}_2$ ​ is 44g/mol
we have 0.05976 moles of $C{O}_2$ ​ since there is 1 mole. so $C$ in $C{O}_2$
we have 0.05976 moles of $C$ or 0.718 g of $C$
mass of $H_{2}O$ = 1.280 g or since the molar mass of water is 18 g/mol
we have 0.07105 mole of $H_{2}O$ since there are 2 moles of $H$ in 1 moles of $H_{2}O$=0.14210 moles of $H$ or 0.143 g of $H$ molar ratio $C:H$ =0.05976 : 0.14210
mass of $C+H$= 0.861 g
or, after divinding by the smallest 0.05976
molar ratio of $C:H$= 1.000 : 2.378
multiply by 3 to get the whole number 3.000 : 7.134

empirical formula is $C_3{H}_7$​

minimum molar mass is $43g/mol$ {Option A}