Question

n this exercise, weighed samples of a solid unknown containing NaCl and NaHCO3 react with hydrochloric acid. The volume of the CO2 liberated by the reaction in the gas phase is measured with a gas collection syringe. From the volume we can calculate the number of moles of CO2 in the gas phase using the ideal gas approximation.

Since the CO2 is generated in an aqueous environment (aqueous HCl), some CO2 will dissolve in the liquid phase. The amount of CO2 dissolved in the liquid phase is computed using Henry's Law which requires knowing the partial pressure of CO2. As described in the exercise, this requires knowing the volume of the entire gas system ( VfTOT)

To insure that your answers agree with Mr. OSCER's, use the values in the below table for the basic constants required. .

	Value	Units
Ideal Gas Constant	0.0821	L-atm/mol K
Convert Celsius temperture to Kelvin	273.15	K
Atmospheric Pressure	760.0	Torr
Molar Mass of NaHCO3	84.01	g/mol
Henry's Law Constant for CO2 in water	3.2 X 10-2	mmol/mL-atm

The figure below illustrates the volumes that are designated by Vtube and Vsyr .

Note also that Vsyr = Vf – Vi .

The table below contains data collected for one run of the required reaction, where an unknown sample mixture containing some NaHCO3 is mixed with hydrochloric acid:

	Value	Units
Initial Weight of container (empty)	18.4395	g
Final Weight of container and unknown	18.6185	g
Volume of hydrochloric acid	10.0	mL
Volume of tube, Vtube (see Figure above)	92.3	mL
Initial volume of syringe   (Vi)	5.0	mL
Final volume of syringe (Vf)	50.7	mL
Temperature	26.5	oC
Atmospheric Pressure	764.4	Torr
Vapor Pressure of Water at 26.5 oC	25.8	Torr

Use this data to answer the following questions. The appropriate units are specified in the each question. Pay close attention to the number of significant figures in your answers.

weight of sample (g) mixture before the start of the reaction = .179 g. Volume of CO2 gas (mL) that is produced by the reaction and is now contained in the syringe= 45.7 mL

1.

How many mmol of CO₂ are present in the liquid (HCl aq) phase ?

Answer 1

solution:

1)

From ideal gas equation, we know: PV = nRT

Where P = pressure of the gas = (Total pressure - vapour pressure )

= (760 - 25.8) torr = 734.2 torr = (734.2/760) atm = 0.966 atm ( 760 torr = 1 atmospheric pressure)

V = volume of the gas = 45.7 mL. = (45.7/1000) Liter = 0.0457 Liter

n = no. of moles of the gas

R = universal gas constant = 0.082 Liter atm mole^-1 K ^-1

T = temperaure = 25.8 ^o C = (25.8 + 273.15) K = 298.95 K

Puttring the values in the equation we get:

n = (0.966 * 0.0457 ) / (0.082 * 298.95) molen

n = 0.00180 moles = 0.00180 * 1000 milimoles = 1.8 millimoles (mmol)

------------------------