urgent chem help?

Question

A sample of zinc metal was reacted with an excess of hydrochloric acid.


Zn(s) + 2HCl(aq) ==> ZnCl2(aq) + H2(g)




All of the zinc reacted, and the hydrogen gas was collected over water at 24C; the barometric pressure was 755 mmHg. The level of water inside the tube was 31.0 cm above the level outside the tube. If the volume of gas in the tube is 21.4 mL, how many grams of zinc were there in the sample? The density of Hg is 13.596 g/mL and the density of water is 1.000 g/mL.

Answer 1

PV = nRT
You're given P, V and T and R is a constant so you're looking for n

Once you get n multiply it by the atomic weight of Zn to get grams

Answer 2

So, you need to figure out the pressure of the gas inside the tube.


31 cm of H20 is the same as 310 mm of H20 which has 1/13.596 the density of Hg therefore its pressure is 22.8 mm Hg

Should we include the effect of water vapor pressure ?, might as well, at 24 C it is 22.1 mm Hg

Therefore the following is true:

755mg Hg = gas pressure + 22.8 + 22.1 mm Hg
gas pressure = 710.1 mm Hg

Lets put that in atm units: 710.1 mmHg*(1atm/760mmHg) =
gas pressure = 0.9343 atm
volume of gas is 0.0214 Liters

pv = nrt, with the gas constant R calculated as:

r = (1atm*22.4 liters)/(1mole*273k) = 0.082 (atm L/mole K)

n = pv/rt = (0.9343atm*0.0214L)/(0.082*297) = 8.21 * 10^-4 mole

From the equation you have 1 mole of Zn for every mole of H2, therefore you had 8.21*10^-4 mole of Zn, How many grams is that?

Please check for errors.