Question

1. hown many grams of Mg must react with HCl in order to produce 80.0 mL of hydrogen gas at STP?
2. when dertermining the pressure of dry hydrogen gas, explain how the measured atmospheric pressure is corrected for the following:
a- the presence of water vapor
b- at level of liquid in the tube higher than in the beacher.
3. what special precautions should be taken when genetating hydrogen gas
4. if 0.071g of magnesium is used in this experiment,what volume of dry H2 gas would theorically be produced if it is measured at 20 celcuis and 785mmHg.
5. what two safery precautions should you take when using concentrated HCl?

Answer 1

1) At STP, 1 mol of any gas has volume of 22.4 Litre = 22400 mL

Mg + 2HCl -----------> MgCl2 + H2

1 mol 1mol

24 g 22400 mL

? 80 mL

? = 24 g Mg x ( 80 mL/ 22400 mL) = 0.0857 g Mg

Therefore,

0.0857 g grams of Mg must react with HCl in order to produce 80.0 mL of hydrogen gas at STP.

4)   

Mg + 2HCl -----------> MgCl2 + H2

1 mol 1mol

24 g 2 g

0.071 g ?

? = (0.071 g / 24 g) x 2 g H2 = 0.006 g H2

Hence,

mass of H2 = 0.006 g H2

Molar mass of the H2 = 2 g/mol

Temperature T = 20 ^∘C = 20 + 273 K = 293 K

Pressure P = 785 mmHg = (785/760) atm

volume V = ?

From Ideal gas equation , PV = n R T , R = 0.0821 L.atm/mol/K

Then, PV = (m/M) RT ( n = moles = mass (m)/ molar mass (M)

V = mRT / MP

= [0.006 g x 0.0821 L.atm/mol/K x 293 K ] / [(2 g/mol) (785/760 atm)]

= 0.07 L

V = 70 mL

Therefore,

volume of H2 gas = 70 mL

5) two safery precautions should you take when using concentrated HCl

   1) Use goggles at all times when handling HCl to protect your eyes and skin.

2) Concentrated hydrochloric acid is toxic if inhaled, so avoid breathing it in and always handle it while under a fume hood.