Specific Heat capacity (1 Viewer)

findsome1

Member
Hey ive got a question im confused with.

A calorimeter containg 25 ml of water was at a steady 18 degrees celcius. 5.85 g of sodium chloride at the same temperature was added to the water and as a result the temperature of the mixture dropped to 15 degrees celcius. Calculate the heat of the solution of sodium chloride under these conditions, i.e the heat change when one mole of sodium cholride is dissolved.

Could you please show the formula used and the working out.

New Member
1. ΔH = - m C Δt (where m is the mass of water in kg (0.025kg), C is the specific heat capacity of water (4.18x10^3) and Δt is the change in temp (-3))

2. ΔH per gram = the answer for above / 5.85g

3. Multiply answer to 2 by molar mass of NaCl (~58.5g)

so i would calculate ΔH/mol = 3135 J (3.135 kJ) and therefore it is an endothermic reaction.

I hope that helps, and if anyone sees a flaw in my method please correct me!

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random-1006

Banned
Hey ive got a question im confused with.

A calorimeter containg 25 ml of water was at a steady 18 degrees celcius. 5.85 g of sodium chloride at the same temperature was added to the water and as a result the temperature of the mixture dropped to 15 degrees celcius. Calculate the heat of the solution of sodium chloride under these conditions, i.e the heat change when one mole of sodium cholride is dissolved.

Could you please show the formula used and the working out.
Q=mc(delta t)

The system is closed and will come to equilbrium, the energy lossed by the NaCl will go into heat ( negatively heating in this case) the water.

now energy lossed by NaCl= (mc delta t) (values for NaCl)

energy ganied by water = - (mc delta t) ( values for water)

one is losing , one is "gaining", thats why the second eqn had negative ( but in this case it gains negative energy)

equating an rearranging

make sure units are consistent, i am converting everything to grams and using c water= 4.186 j/ (g C)
c (NaCl) = - ( m water) ( c water) ( delta t) / [ ( m NaCl) (delta t) ]
= - ( 25) ( 4.186) (-3) / [ (5.85) *(-3) ]

(the answer will have units J/ (g C) [ this will give heat transfered per gram ]

then we know molar mass is the mass of one mole (M= m/n), n= m/M

so if we divide the above answer by molar mass of NaCl which is approx 58.5 as someone else said we will get answer per mole

evaluate that, im pretty sure it is correct, the answer is negative which is what it should be, as the temperature decreased rather than increased

lucky i did thermodynamics this sesmester at uni, im not sure if i remember doing it like that in yr11, 2 years, its been too long lol

the answer i get is very small though, i dont think its right somehow, but i dont know where it would be wrong

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random-1006

Banned
1. ΔH = - m C Δt (where m is the mass of water in kg (0.025kg), C is the specific heat capacity of water (4.18x10^3) and Δt is the change in temp (-3))

2. ΔH per gram = the answer for above / 5.85g

3. Multiply answer to 2 by molar mass of NaCl (~58.5g)

so i would calculate ΔH/mol = 3135 J (3.135 kJ) and therefore it is an endothermic reaction.

I hope that helps, and if anyone sees a flaw in my method please correct me!

im not 100% sure thats correct, i dont think they would have the same change in energy per gram

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