Calculate the molarity of Na+ ions in 20 mL of a .130 M solution of Na2Cr2O7?
$0.0052$ is the number of moles of $\ce{Na+}$ ions in solution, but to find the molarity, you need to divide by $\pu{0.02 L}$. Therefore, the final answer is $\ce{2\cdot 0.130 \cdot 0.02 \cdot \frac{1}{0.02} = 0.260 M}$