Question

A vessel contains 14 g of hydrogen and 96 g of oxygen at STP. Chemical reaction is induced by passing electric spark in the vessel till one of the gases is consumed. The temperature is brought back to its starting value 273 K. Find the pressure in the vessel.

• A. 0.1 atm
• B. 0.3 atm
• C. 0.2atm
• D. 0.4 atm

Answer 1

The correct option is A

0.1 atm

The number of moles of hydrogen

$=\left[\frac{14g}{2g}\right]=7moles$

Similarly,number of moles of oxygen $=\left[\frac{96g}{32g}\right]=3moles.$

$\therefore$ Total number of moles in the container before the reaction = 10 moles of gases.

Also, the pressure Pbefore the reaction = 1 atm (STP). When the electric spark is passed, two moles of hydrogen combine with one mole of oxygen to form one mole of water. After the reaction is complete, we will be left with only one mole of hydrogen (since the 3 moles of oxygen will bond with 6 moles of hydrogen to make 3 moles, or 42 mL of water). Ignoring the volume occupied by the water, decreased pressure P will be -

$P\prime V=RT$

[$n\prime =1$ in this case since the container will have only one mole of gas].

Initially we had at STP

PV=nRT

Dividing (1)by (2) , we get

$\frac{P\prime }{P}=\frac{n\prime }{n}$

$\Rightarrow P\prime =\frac{Pn\prime }{n}=\frac{1\times 1}{10}atm=0.1atm.$