Heat capacity problem-wher am i going wrong?

[debwaldy]

Homework Statement

Hi,so iv tried the following question and think my general methodology is correct but the final answer i am getting is wrong.ne ideas?

Homework Equations

Question:a styrofoam cup contains 200g of water at 20 degrees celsius.What will be the equilibrium temp of the system after 100g of silver,initially at 300 degrees,has been added?you may assume that the heat capacity of the cup is negligible and that no heat is lost to the surroundings.

(b)a further 100g of an unknown substance,also at 300 degrees is now added to the system and the temperature rises by a further 26 degrees celsius
what is the specific heat capacity of the unknown substance?(the specific heat of water is 4180 Jkg^-1 K^-1 and that of silver 240 J kg^-1 K^-1)

The Attempt at a Solution

so i got part a to work out correctly by saying:
heat energy lost by silver = heat energy gained by water
mass(silver).c(silver).(initial temp-final temp) =mass(water).c(water).(final temp-initial temp)

i.e. 0.1(240)(300-final temp)=0.2(4180)(final temp-20)
and found the equilibrium temp=28 degrees celsius,which i know is correct

its part b I am having trouble with:the correct answer is supposed to be 910 J kg^-1 K-1 but i keep getting 8600,by using the following,is it correct:

heat energy lost by unknown substance= heat energy gained by water and heat energy gained by silver

then:
mass(unknown).C(unknown).(26)=mass(water).c(water).(26) + mass(silver).c(silver).(26)

i.e. 0.1(c)(26) = 0.2(4180)(26)+0.1(240)(26)

=> c= 8600..which is wrong according to book?
any help wud be much appreciated
thanks

 

[rsk]

What is the temperature change for the unknown mass?[What is the final temperature for all three? = 28 + 26]

Using this, and your method, I get 908.9, which is close enough for me.

 

[debwaldy]

ok you that seems to make a lot more sense alright
thanks for the help