Homework Help: Chemistry

Posted by Anonymous on Monday, February 25, 2008 at 1:01pm.

When 0.655 g of Ca metal is added to 200.0 mL of 0.500 M HCl(aq), a temperature increase of 107C is observed. Assume the solution's final volume is 200.0 mL, the density is 1.00 g/mL, and the heat capacity is 4.184 J/gC. (Note: Pay attention to significant figures. Do not round until the final answer.)

The molar heat of reaction, H rxn, for the reaction of

Ca(s) + 2H+(aq) Ca2+(aq) + H2(g)

is ______ kJ/mol.

---
I used q=m*s*DeltaT
q=(200)(4.184)(10.7)
For mols, I got 0.0163.
I multiplied the two & got it wrong. I don't know what to do from there.

Chemistry - DrBob222, Monday, February 25, 2008 at 1:15pm
This question has floated around on this board for 2-3 days. I posted one response that I couldn't understand delta T. The numbers I have seen are 107, 102, >200, and >300 but none of these make sense to me because anything over 100 means we convert water to steam. No one has addressed those issues.
With regard to your latest post, I notice the delta T is 107 BUT your calculation uses 10.7. Which is correct? IF it truly is 10.7 then your q is calculated correctly. The answer will be in Joules. Then to find the Joules/mol you must DIVIDE by (not multipy) by the mols which you show correctly as 0.0163. Then you will need to change that to kJ/mol
Chemistry - Anonymous, Monday, February 25, 2008 at 4:43pm
It is 10.7. When I copied & pasted the question, the decimal must have not pasted correctly.

This is what I did:
q=(200)(4.184)(10.7)
q=8953.76
mols = 0.655/40.078
mols = 0.0163
J/mol = 8953.76/0.0163 = 549310.4294
549310.4294/1000 = approx. 549

I entered 549, 550, & 549.31 and still got it wrong. Am I missing something?
Chemistry - Anonymous, Monday, February 25, 2008 at 4:49pm
Btw, thanks for the help.
Chemistry - DrBob222, Monday, February 25, 2008 at 5:19pm
I multipled
200.0 x 4.184 x 10.7 x (1/1000)and divided by (0.655/40.078) and obtained 547.86075 which rounds to 548 kJ/mol to 3 s.f. (the 0.655 g has only 3 s.f. unless you omitted a final zero). So I don't think you've missed anything except you rounded the 0.0163 mols (0.655/40.078 = 0.0163431) and the problem specifically states to round AT THE END. So I left all of those numbers in my calculator and rounded at the end. Try 548 kJ/mol.
Chemistry - Anonymous, Sunday, February 22, 2009 at 1:46pm
Hello,
Try making your answer negative.
Chemistry - Anonymous, Monday, February 23, 2009 at 3:25am
yes, make your answer negative

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