Calculate the no. of molecules present in 0.5 moles of CO2?

The molar mass of CO₂ is 44 g. 
Which means, the mass of 1 mole of CO₂ is 44 g 
1 mole = 44 g 
0.5 mole = 44 $\times$ 0.5 g 
Hence the mass of 0.5 mol of CO₂ gas is 22g 
As 1 mole contains 6.022 $\times 10{ }^{23}$ molecules of CO₂, 
$$\begin{gathered} \therefore 0.5 \mathrm{mole} \mathrm{of} {\mathrm{CO}}_2 \mathrm{contains} = 6. 022 \times 10{ }^{23 } \times 0.5 \\ = 3.011 \times 10{ }^{23 } \\ \mathrm{Hence}, 0.5 \mathrm{mol} \mathrm{of} {\mathrm{CO}}_2 \mathrm{contains} 3.011 \times 10{ }^{23 } \mathrm{molecules}. \end{gathered}$$