Question

			used Mg for these trials
Trial	HCl (mL)	Mass (g)	Ti degrees Celsius	Tf degrees Celsius
#1	7.5 mL	0.08 g	24 o C	62.0 o C
#2	7.5 mL	0.05 g	24.9 o C	57.9 o C
			used MgO for these trials
Trial	HCl (mL)	Mass (g)	Ti degrees Celsius	Ti degrees Celsius
#1	7.5 mL	0.13 g	23.3 o C	30.1 o C
#2	7.5 mL	0.15 g	22.7 o C	36.8 o C

T_i = initial temperature T_f = final temperature

Report the calculated value for ∆H_f° for MgO and the percent error, and why can the specific heat capacity of the HCl solution be assumed the same as the specific heat capacity as water?

Answer 1

For trial 1.:

Total mass of solutio --> 7.5 mL = 7.5 g

mol of Mg = mass/MW= 0.08 / 24.3 = 0.0033 mol of Mg

1 mol of Mg --> 1 mol of MgO

dT = Tf-Ti = 62-24 = 38 °C

Qsolution = m*C*dT = 7.5*4.184*38 = 1192.44 J

now

HRxn = -Q/n = -1192.44 / 0.0033

HRxn = -361345.45 J/mol

HRxn = -361.35 kJ/mol

The same is valid for all other species

Actual value for MgO = -601.6 kJ/mol

Note that

%error = (real -calcualted)/(real ) * 100% = (361.35  - 601.6) / 601.6*100 = 39.9%

now...

we can assume that Hcl solution has same properties as water, since it is diluted

note that HCl is approx 2-5% in solutoin vs. 90-95% of water

therefore, Speicifc heat of solution = Speicifc heat of water