In 2 Na(s) + 2 H2O(l) 2 NaOH(aq) + H2(g), the hydrogen gas generated is collected over water at 25.0°C. The volume of the gas is 200 mL measured at 1.00 atm. What is the number of grams of sodium used? Vapor pressure of water at 25°C = 0.0313 atm.

Jun 24, 2018

We need to assess the molar quantity of gas...we get approx. $400 \cdot m g$ of sodium...

Explanation:

….for which we use the Ideal Gas equation...

${n}_{{H}_{2}} = \frac{P V}{R T} = \frac{\left(1.00 - 0.0313\right) \cdot a t m \times 0.200 \cdot L}{0.0821 \cdot \frac{L \cdot a t m}{K \cdot m o l} \times 298 \cdot K} = 7.92 \times {10}^{-} 3 \cdot m o l$...

We have SUBTRACTED ${P}_{\text{saturated vapour pressure}}$ from the laboratory pressure....the gas is collected over water...

And we assess the reaction...

$N a \left(s\right) + {H}_{2} O \left(l\right) \rightarrow \frac{1}{2} {H}_{2} \left(g\right) + N a O H \left(a q\right)$

And thus there are 2 equiv of sodium metal...

$2 \times 7.92 \times {10}^{-} 3 \cdot m o l \times 22.99 \cdot g \cdot m o {l}^{-} 1 = 0.364 \cdot g$