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Calculate the final temperature of the mixture

  • #1

Homework Statement


Consider the following reaction.
2 HCl(aq) + Ba(OH)2(aq)
rtarrow.gif
BaCl2(aq) + 2 H2O(l) ΔH = -118 kJ
Calculate the heat when 100.8 mL of 0.500 M HCl is mixed with 300.0 mL of 0.450 M Ba(OH)2. Assuming that the temperature of both solutions was initially 25.0°C and that the final mixture has a mass of 400.8 g and a specific heat capacity of 4.18 J/°C · g, calculate the final temperature of the mixture.

Homework Equations


Q=mc(delta)T
c=m/V

The Attempt at a Solution


using c=m/V and that n = m/M, I got that HCl is the Limiting Reactant and that there are 0.001382 moles of it. Then Q = delta H = -118*0.001382/2 = -0.081538
Plugging this into Q=mc(delta)T:
-0.081538 = 400.8 x 4.18 x (T_f - 25)
I got T_f = 24.99..., which is wrong.

Where did I go wrong? My assignment is due soon, so please help!
 

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Answers and Replies

  • #2
mjc123
Science Advisor
Homework Helper
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437
Where do you get 0.001382 moles from?
You have failed to convert kJ into J.
Negative ΔH means heat is given out, so that raises the temperature - Q in Q = mcΔT should be positive.
 
  • #3
@mjc123
.1008L x 0.5 M = 0.0504 g
and since the molar mass of HCl is (35.45 + 1.008)g/mol, doing 0.0504/(35.45+1.008) = 0.001382
am I doing it wrong?
 
  • #4
@mjc123 I tried to convert kJ into J (supposing the moles is right above), but for some reason, I get a similar answer — 25.0487 — but is this right?
 
  • #5
Borek
Mentor
28,352
2,736
.1008L x 0.5 M = 0.0504 g
No, that's not how it works. You don't have 0.0504 g o the substance.

What does 'M' mean in the context of the concentration?
 
  • #6
@Borek
M means molar, which is concentration.
Oh the equation was in fact c= n/V, not m/V... right?
 
  • #7
@Borek
Would the answer then be 26.7749 °C (ignoring sigfigs for now)?
 
  • #8
Borek
Mentor
28,352
2,736
Looks much better.
 

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