An unknown sample contains 0.2618 mol of MgCO3. How many atoms of oxygen?

Answer 1

One mole of substance contains 6.022*10^23 molecules of that substance (Avogadro's number). 0.2618 moles then contains
0.2618*6.022*10^23 molecules of MgCO3. Each molecule of MgCO3 contains 3 oxygen atoms. Therefore there are

3*0.2618*6.022*10^23 = 4.73*10^23 atoms of oxygen in the sample