At STP, one mole of any gas occupies 22.4 liters. Therefore, 2.8375 liters of oxygen gas at STP is equal to 0.125 moles. The number of molecules of oxygen gas in 0.125 moles is calculated by multiplying the number of moles by Avogadro's number, which is $$6.022 × 10^{23} $$ molecules/mol. This gives us a total of $$7.527 × 10^{22}$$ molecules of oxygen gas in 2.8375 liters at STP.

Here is the calculation:

$$\begin{align} &\text{Number of moles of }O_2 = \frac{2.8375 \text{ L}}{22.4 \text{ L/mol}} = 0.125 \text{ mol} \\\\ &\text{Number of molecules of }O_2 = 0.125 \text{ mol} \times 6.022 \times 10^{23} \text{ molecules/mol} = 7.527 \times 10^{22} \text{ molecules} \end{align} $$