Question

Complete combustion of 0.858 g of compound X gives
2.63 g of CO2 and 1.28 g of H20. The lowest molecular
mass X can have

a) 43 g
(b) 86 g
c) 129 g
(d) 172 g​

Answer 1

$\bold{\underline{QUESTION}}$

Complete combustion of 0.858 g of compound X gives 2.63 g of CO2 and 1.28 g of H20. The lowest molecular mass X can have?

$\bold{\underline{ANSWER}}$

$\mapsto$ Mass of Carbon in 2.63 g of ${CO}_{2}$ =

$2.63 \times \frac{12}{44} = 0.717 \: g$

$\mapsto$ Mass of hydrogen in 1.28 g of ${H}_{2}{O}$ =

$1.28 \times \frac{2}{18} = 0.142 \: g$

Percentage of Carbon =

$0.717 \times \frac{100}{0.858}$

= 83.6%

Percentage of Hydrogen =

$0.142 \times \frac{100}{0.858}$

= 16.6%

The compound X can only be a hydrocarbon as the total percentages of carbon and hydrogen is almost equal to 100%.

$\mapsto$ C:H = 83.6/12 : 16.6/1

= 6.97 : 16.6

= 1 : 16.6/6.97

= 1 : 2.38

On converting to the whole number ratio, 1 : 2.38 = 5 : 12.

So, the formula of the hydrocarbon is C5H12.

Molecular mass of X = 12 × 5 + 1 × 12

= 72 [ANSWER]

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