4. Determine the number of molecules that are in 1.25 moles of O_(2)

The question is asking for the number of molecules in 1.25 moles of $O_{2}$. To find this, we need to use Avogadro's number, which is $6.022 \times 10^{23}$ molecules per mole. This number represents the number of molecules in one mole of any substance. <br /><br />To find the number of molecules in 1.25 moles of $O_{2}$, we multiply the number of moles by Avogadro's number:<br /><br />\[1.25 \text{ moles} \times 6.022 \times 10^{23} \text{ molecules/mole} = 7.5275 \times 10^{23} \text{ molecules}\]<br /><br />So, there are $7.5275 \times 10^{23}$ molecules in 1.25 moles of $O_{2}$.