Posted by Rebekah on .
Calculate the amount of heat required to raise the temperature of a 25 g sample of water from 5°C to 10.°C.
i got 523 then rounded it to two sig figs
520 but it said it was wrong

chem. 
DrBob222,
Good. You rounded BUT 520 is to three s.f. I would round it to two but write it as 5.2 x 10^2 J. Are you sure it's two s.f.? 105 is to only one s.f. which would be 5 x 10^2 J.

chem. 
Rebekah,
thanks!

chem. 
Rebekah,
i have another question:
A 43 kg sample of water absorbs 343 kJ of heat. If the water was initially at 22.1°C, what is its final temperature?
I got 1.9 but i think i didn't do it correctly 
chem. 
DrBob222,
1.9 is the difference in TfinalTinitial so Tfinal must be 24.0 (24.022.1 = 1.9).
343,000 = 43,000 x 4.184 x (Tf22.1) = 0
Solve for Tf which I get as 24.0. 
chem. 
hannah,
thank you so much!!

chem. 
hannah,
oh, would it be 24 (two sig figs) cause in the problem 43 has the least number of sig figs(2) or do we go by this equation:
343,000 = 43,000 x 4.184 x (Tf22.1) = 0
and use 3 sig figs? 
chem. 
Ms. Sue,
Hannah, Rebekah  it's hard to remember which name to use when you use more than one, isn't it?
Please stick with just one name in this forum. 
chem. 
DrBob222,
I think you are right. 43 has only two s.f. so the answer should be 24. Technically we don't know how many s.f. are in 43 kg. If they meant two it should have been written as 4.3 x 10^1; still, I would go with two and round the final answer to 24.

chem. 
DrBob222,
And Ms. Sue has a very good point. I saw the same question above under Hannah and thought I had a different student (until I came back here to copy the answer I had given to Rebekah).