Question

A vessel contains 1.6 g of dioxygen at STP (273.15K, 1 atm pressure). The gas is now transferred to another vessel at constant temperature, where pressure becomes half of the original pressure. Calculate number of molecules of dioxygen.

Answer 1

The number of moles of dioxygen is calculated by the formula,

Moles = `"Mass"/"Molar mass"`

Substitute all the known values in the above equation as follows,

Moles of \[\ce{O2}\] = `(1.6  g)/((32  g)/(mol)` = 0.05 mol

1 mole of dioxygen contains `6.022 xx 10^23` molecules. So, 0.05 moles will contain as follows,

Molecules of \[\ce{O2}\] = `(6.022 xx 10^23)/(1  mol) xx 0.05  mol = 3.011 xx 10^22` 

Therefore, the number of molecules of dioxygen is `3.011 xx 10^22`.