Question

If I threw a block of sodium into a lake, and the H2 created took up 34 L of space at STP (standard temperature and pressure: 1 atm and 0C), how many grams of Na did I begin with?

Answer 1

Answer:

70g Na

Explanation:

The sodium reacts with water as follows:

2Na + 2H2O → 2NaOH + H2

Where 2 moles of Na produce 1 mole of H2.

To solve this question we must find the moles of H2. With the balanced equation we can find the moles of Na and its mass as follows:

Moles H2:

PV = nRT; n = PV/RT

Where P is pressure = 1atm,

V is volume = 34L,

R is gas constant = 0.082atmL/molK

T is absolute temperature = 273.15K = 0°C

Replacing:

n = 1atm*34L/0.082atmL/molK*273.15K

n = 1.52 moles H2

Moles Na:

1.52 moles H2 * (2mol Na / 1mol H2) = 3.04 moles Na

Mass Na-Molar mass: 22.99g/mol-

3.04 moles Na * (22.99g / mol) =

70g Na