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.
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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.
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
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?
Btw, thanks for the help.
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.
Hello,
Try making your answer negative.
yes, make your answer negative
To find the molar heat of reaction (Hrxn), you need to calculate the quantity of heat (q) released during the reaction and then convert it to kJ/mol.
First, calculate the quantity of heat (q) using the equation q = m × s × ΔT, where:
- m is the mass of the solution (in grams),
- s is the specific heat capacity of the solution (in J/g°C), and
- ΔT is the change in temperature (in °C).
You already correctly calculated q as 200 g × 4.184 J/g°C × 10.7 °C = 8596.8 J.
Now, you need to convert the mass of the solution (200 g) into moles. To do that, you'll first find the moles of Ca metal reacted.
The molar mass of Ca is 40.08 g/mol. Therefore, the moles of Ca reacted are:
moles of Ca = mass of Ca / molar mass of Ca
= 0.655 g / 40.08 g/mol
= 0.0163 mol
Since the balanced equation for the reaction shows that 1 mol of Ca reacts with 2 mol of H+, the moles of H+ reacted are twice the moles of Ca reacted:
moles of H+ = 2 × moles of Ca
= 2 × 0.0163 mol
= 0.0326 mol
Now, you can calculate the molar heat of reaction (Hrxn) using the equation:
Hrxn = q / moles of H+
Plugging in the calculated values:
Hrxn = 8596.8 J / 0.0326 mol
= 263867 J/mol
Lastly, convert the value to kJ/mol by dividing by 1000:
Hrxn = 263.867 kJ/mol
Therefore, the molar heat of reaction (Hrxn) for the given reaction is 263.867 kJ/mol.