March 27, 2017

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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
For mols, I got 0.0163.
I multiplied the two & got it wrong. I don't know what to do from there.

  • Chemistry - ,

    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 - ,

    It is 10.7. When I copied & pasted the question, the decimal must have not pasted correctly.

    This is what I did:
    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 - ,

    Btw, thanks for the help.

  • Chemistry - ,

    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 - ,

    Try making your answer negative.

  • Chemistry - ,

    yes, make your answer negative

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