Determination of the gas constant

PV = nRT

- 1electronic scale to 3 decimal places.
-100mL measuring cylinder
- copper wire and magnesium ribbon
- barometer, thermometer
- 250 mL beaker.
- wire cutters
- 2M HCl

Caution - Hydrogen gas is explosive and must not be allowed to accumulate. This activity should be conducted in the absence of naked flames. Goggles, overcoat and gloves should always be worn by students.



1) Weigh a set of magnesium ribbons of approximately 0.05 grams and place each one in an individual plastic bag witht eh mass recorded. Most labs have only one three decimal place scales and as such student access is limited. By weighing out a set of magnesium ribbons before the class the need for student access to expensive equipment is removed.



2) Cut some copper wire and construct a cross hair at the opening of the measuring cylinder, as shown on the right. Secure the magnesium ribbon in the middle of the cross hairs.


3) Fill the measuring cylinder with water and place the beaker over the top as shown on the right.


Carefully invert the measuring cylinder and beaker. The water should remain in the measuring cylinder.


4) Record the level of the water in the measuring cylinder.


5) Pour 2.0 M HCl in the beaker. The reaction between magnesium and HCl should now take place producing hydrogen gas according to the equation below.

Mg(s) + 2HCl(aq) => MgCl2(aq) + H2(g)


Bubbles of hydrogen gas should rise displacing the water in the measuring cylinder.


6) Record the temperature of the gas and convert it to Kelvin. The reaction is exothermic, heat producing, so measure the temperature of the water to get a better indication of the temperature of the gas.


7) Finally record the air pressure in the room. Convert it to kPa.

i) Calculate the mol of magnesium used. The mass of the magnesium should be written on the plastic bag containing the sample of magnesium.

ii) The hydrogen gas sample will not be pure, it will contain some water vapour. Subtract the partial pressure of water from the air pressure reading. The partial pressure of water at:

20 oC is 2.3 kPa
22 oC is 2.6 kPa
24 oC is 3.0 kPa
26 oC is 3.4 kPa
28 oC is 3.8 kPa

Why must we take the partial pressure of water vapour into account?
what would be the value of the gas constant if the partial pressure of water was not taken into account? Higher, Lower, Unchanged

iii) Calculate R to two decimal places, using PV/nT = R

iv) If the accepted value of R is 8.31 what is your percentage error?