Question

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

(A) 0.0675g Mg AND 4mL of 6M HCl were used. Barometric pressure were 28.85 inches Hg. A volume of 73.6mL of hydrogen was collected at a temperature of 26.0C. The vapor pressure of water is 25.2mm Hg at this temperature. Calculate R, the gas law constant, in L-atom\mol-K using the information above for the reaction of HCl and Mg. 760 mmHg= 1.00atm. 1 inch=2.54 cm.

Show your calculation

(B) why is it necessary to substract the vapor pressure of the hydrogen gas formed to your experiment?

(C) If some gas was lost during the process and the measured amount of gas was less than the amount that should have been produced, would this increase or decrease the calculated R ? Explain your answer.

Answer 1

Solution :-

a) Lets calculate the moles of the hydrogen gas produced from each reactant

(0.0675 g Mg * 1 mol / 24.305 g ) *(1 mol H2/1 mol Mg) = 0.002777 mol H2

Moles of HCl = molarity * volume

                         = 6.0 mol per L * 0.004 L

                          = 0.024 mol HCl

0.024 mol HCl * 1 mol H2/2 mol HCl = 0.012 mol H2

So the limiting reactant is Mg so the moles of H2 gas formed are 0.002777 mol

Now lets calculate the pressure of the hydrogen gas

28.85 in Hg * 25.4 mmHg / 1 in = 733 mmHg

Pressure of H2 = total pressure - vapor pressure of water

                          = 733 mmHg – 25.2 mmHg

                          = 707.8 mmHg

Volume of H2 gas = 73.6 ml * 1 L / 1000 ml = 0.0736 L

Temperature = 26.0 C +273 = 299 K

Now lets calculate the gas constant R

PV= nRT

R= PV/ nT

   = (707.8 mmHg * 1 atm /760 mmHg)*0.0736 L / (0.002777 mol * 299 K)

   = 0.082552 L atm per mol K

So the value of the gas constant R obtained is 0.082552 L atm per mol K

(B) why is it necessary to substract the vapor pressure of the hydrogen gas formed to your experiment?

Solution :- The total pressure in the flask is due to the parital pressure of water vapor and partial pressure of H2 gas so to get the exact pressure of the H2 gas we ned to substract the vapor pressure of water so that we can find the exact pressure of the H2 gas.

(C) If some gas was lost during the process and the measured amount of gas was less than the amount that should have been produced, would this increase or decrease the calculated R ? Explain your answer

Solution :- If the some of the gas is lost during the process of the measurement of the gas formed then it will give the higher value of the R because the moles of gas is inversely proportional to R so if n is less than actual then value of the R would be higher.