Question

Mass of flask, stopper, and 5mL of water	102.945	g
Mass of flask, stopper, and filled with water	238.389	g
Volume of gas space in flask (the difference between the above two measurements converted to mL) (L)	.135444	L

NaHCO₃ + HCl reaction

	Trial 1	Trial 2	Units
Mass of NaHCO3	0.053	0.050	g
Mass of system before reaction	117.026	117.189	g
Mass of system after reaction	116.809	117.159	g
Mass Change	0.217	0.030	g
Initial Pressure	0.9876	0.9887	atm
Maximum Pressure	1.0515	1.0648	atm
Change in Pressure	0.0639	0.0761	atm
Temperature of water bath	296.3	296.6	K

a)From the P,V,T data calculate the ACTUAL moles of CO₂ produced in each trial

b) With your result from part a, calculate the molar mass of CO₂ for each trial.

I think I'm using the wrong pressure in my calculations when I try it. I'm not sure :(

Answer 1

Mass of flask + stopper + 5 mL water (g)	102.945
Mass of flask + stopper + filled with water (g)	238.389
Volume of gas space in the flask (L)	0.135444

	Trial 1	Trial 2
Mass of NaHCO3 (g)	0.053	0.050
Mass of system before reaction (g)	117.026	117.189
Mass of system after reaction (g)	116.809	117.159
Mass change (g)	0.217	0.030
Initial pressure (atm)	0.9876	0.9887
Maximum pressure (atm)	1.0515	1.0648
Change in pressure (atm) = (maximum pressure – initial pressure)	0.0639	0.0761
Temperature of water bath (K)	296.3	296.6
Moles of CO2 produced	0.0003562 (see sample calculation 1 below)	0.0004238
Molar mass of CO2 (g/mol)	609.208 (see sample calculation 2 below)	70.788

Sample calculation 1:

Pressure of CO₂, i.e., P_CO2 = maximum pressure – initial pressure.

This is due to the fact that when the reaction takes place, CO₂ is the only gaseous product that is formed and the formation of CO₂ gas increases the pressure of the system as per the below equation:

NaHCO₃ (aq) + HCl (aq) ------> NaCl (aq) + CO₂ (g) + H₂O (l)

Volume of CO₂ gas = 0.135444 L (since CO₂ fills up the entire space in the flask).

Use the ideal gas law to find the moles of CO₂ produced as

n = P_CO2.V/RT = (0.0639 atm).(0.135444 L)/(0.082 L-atm/mol.K).(296.3 K) = 3.562*10^-4 = 0.0003562 mol

Sample calculation 2:

Mass change in the above reaction occurs because CO₂, being a gas, escapes from the system and hence, the final mass of the products after the reaction is lower than the initial mass. Hence, mass change = mass of CO₂ formed.

Therefore, mass of CO₂ formed = 0.217 g

Molar mass of CO₂ = (mass of CO₂ formed)/(moles of CO₂ formed) = (0.217 g)/(0.0003562 mol) = 609.208 g/mol

Offcourse, the molar masses you obtain following this procedure is grossly off. The reason is the mass change on heating. You have obtained a much higher mass change and this is possibly because you expelled water vapor from the system as well. The procedure you are following is correct; however, re-check the experimental values again.