How many molecules of carbon dioxide are there in 44.0 grams of the gas? (atomic mass of carbon = 12.0 and atomic mass of oxygen = 16.0)

Question

unklerhaukus · Answer

The molecular mass $M$ is the sum of atomic masses of the atoms in the molecule.

For carbon dioxide; CO_2, 

$$M_{\text{CO}_2}=M_\text C+2M_\text O\\qquad\quad=12.0\,[\tfrac{\text g}{\text{mol}}]+2\times16.0\,[\tfrac{\text g}{\text{mol}}]\%\qquad\quad=44\,[\tfrac{\text g}{\text{mol}}]$$

Use the molecular mass of CO_2 to convert mass, to a number of moles $n$
$$M\triangleq \frac mn\implies n=\frac mM$$

Avogadro's number $N_A$ is the number of  particles $N$ in one mole of any substance,

so simply multiply the number of moles $n$ you found above by Avogadro's number
$$N_A=6.022\times10^{23}\,[\text{particles}/\text{mol}]$$

$$N=nN_A$$