Question

Part 1. using the ZN sample mass (1.948), calculate the number of moles of ZN (m) used in the reaction. NOTE: in the experiment it said to dispense approximately 2 grams of ZN and 10 ml HCl

Part 2. Using the number of moles of Zn, and the mole ratio in the reaction,

Zn (m) + 2 HCl (aq) - ZnCl₂ (aq) + H₂ (g)

determine the number of h2 produced

Part 3. The pressure observed after reaction with Zn metal and HCl should be corrected to subtract the pressure due to water vapor present at the final system temperature (22 degrees C).

Enter the corrected pressure P_corr =P_abs - P_h2o below

Answer 1

Part 1 - The number of moles of Zn = Zn sample mass / atomic mass of Zn

= 1.948 / 65,39

= 0.02979 moles

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Part 2 - The stoichiometric ratio of the reaction is studied and represented as below -

Zn (m) + 2HCl -------------> ZnCl₂ (aq) + H₂ (g)

1 mole 2moles 1 mole 1 mole

1.00 mole of Zn reacts to give out 1.00 mole of H2 gas

0.02979 moles of Zn would yield 0.02979 moles of H₂  gas

or 0.03 moles

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Part 3 - One mole of Hydrogen gas at NTP occupies 22,4 litres of volume

therefore 0.03 moles ---------------------------------------------( 22.4 x 0.03 ) = 0.672 litres

Thus, 0.672 litres of hydrogen gas are liberated by the reaction at 273 K, hence we need to convert this volume at 293 K (of course at atmospheric pressure ie. 1 bar ) using Charle' relation-

V1/T1 = V2 /T2

0.673 /273 = V2 / 295

V2 = 0.726 litres

Now , find out the pressure exerted by 0.726 litres of this gas using relation P1 V1 /T1 = P 2 V2 /T 2

Apply the relation - P1V1 /T1 = P2V2 / T 2 to get the pressure of the gas at 22 degrees centigrade or 295K

(1bar) ( 0.672 ) /273 = P 2 ( 0.726 ) / 295

P 2 = (1 bar) (0.672)( 295) / 0.726 x273

= 1.0002 bar

or, = 1.00 bar

but as per conditions pressure due to water vapour ( ie. aq tension ) is to be subtracted, hence-

P corrected = 1.00 - 0.03*

= 0.97 bar

Again, since 1 bar is nearly identical to atm. & 1bar = 750.062 torr

the corrected pressure in torr unit = 750.062 x 0.97

= 727. 56 torr

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Note : since the strength or normality of HCl solution is not given with the remarks that 10 ml. of HCl is able to dispense the given mass of zinc, the reaction should go to completion under the conditions.

*Aq .tension of water at 295K = 0.03 bar