# chem enthalpy change exam question

this may be a stupid question but why is the mass that is used in the formula for temperature change 100g and not 50g
mass using density is density x volume so shouldnt it be 1.00x50?

this is the question btw

In a different experiment 50.0 cm3 of 0.500 mol dm–3 aqueous hydrochloric acid are
reacted with 50.0 cm3 of 0.500 mol dm–3 aqueous sodium hydroxide.
NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) ΔH = –57.1 kJ mol–1
The initial temperature of each solution is 18.5 °C
Calculate the maximum final temperature of the reaction mixture.
Assume that the specific heat capacity of the reaction mixture, c = 4.18 J K–1 g–1
Assume that the density of the reaction mixture = 1.00 g cm–3

The markscheme has this as the answer:

n(HCl) or n(NaOH) = 50 x 0.500 / 1000 = 0.025 moles
q = –ΔH x n = 57.1 x 0.025 = 1.4275 kJ
ΔT = q/mc
ΔT = (1.4275 x 1000) / (100 x 4.18) = 3.4(2) °C
Final Temperature = 18.5 + 3.4 = 21.9 °C
Original post by dnmki
this may be a stupid question but why is the mass that is used in the formula for temperature change 100g and not 50g
mass using density is density x volume so shouldnt it be 1.00x50?

this is the question btw

In a different experiment 50.0 cm3 of 0.500 mol dm–3 aqueous hydrochloric acid are
reacted with 50.0 cm3 of 0.500 mol dm–3 aqueous sodium hydroxide.
NaOH(aq) + HCl(aq) NaCl(aq) + H2O(l) ΔH = –57.1 kJ mol–1
The initial temperature of each solution is 18.5 °C
Calculate the maximum final temperature of the reaction mixture.
Assume that the specific heat capacity of the reaction mixture, c = 4.18 J K–1 g–1
Assume that the density of the reaction mixture = 1.00 g cm–3

The markscheme has this as the answer:

n(HCl) or n(NaOH) = 50 x 0.500 / 1000 = 0.025 moles
q = –ΔH x n = 57.1 x 0.025 = 1.4275 kJ
ΔT = q/mc
ΔT = (1.4275 x 1000) / (100 x 4.18) = 3.4(2) °C
Final Temperature = 18.5 + 3.4 = 21.9 °C

When you mix together 50g of one component and 50g of the other component how much mass do you have?
You always use the mass of the surroundings rather than the mass of the individual thing you're investigating.
Original post by charco
When you mix together 50g of one component and 50g of the other component how much mass do you have?

thank u, i ended up figuring it out.