[Bronx19]

Hey guys, I just received the results of an exam and would like to know the correct working an answer to a few questions:

Q1) A solution of acid is to be neutralized. The volume of the solution is 1500L, the concentration of HCl is known to be 0.84 mol/L by analysis. What mass of Ca(OH)2 in KG is needed to neutralize the acid?

My answer was 93Kg,and incorrect.

Q2) A car tyre is inflated to a total pressure of 188KPa with air at 23.7 degree's C. After five hours of driving the volume has increased by 2.8% and the total pressure is 206KPa. Calculate the temperature in C of the air in the tyre to three sig figs.

Thanks for any advice.

[Borek]

1. Your answer is incorrect.

2. You have not provided any answer, thus it is hard to tell whether your working was OK.

Show how you approached both problems.

[Bronx19]

Thanks for the reply.

In regards to the first question, is it possible that the concentration of OH is 2 mol, making the final answer 47Kg?

For the second question, I used T2 = T1 (P1/P2). Getting a final answer of 25.97C.

[Yggdrasil]

Thanks for the reply.

In regards to the first question, is it possible that the concentration of OH is 2 mol, making the final answer 47Kg?

Yes.  In order to avoid mistakes like this in the future it is useful to write out the balanced chemical reaction:

2 HCl + Ca(OH)₂ --> CaCl₂ + 2 H₂O

which makes it clear that for every 2 moles of HCl, you need only one mole of calcium hydroxide.

For the second question, I used T2 = T1 (P1/P2). Getting a final answer of 25.97C.

This equation assumes volume is constant, but volume is changing in the question.  It is much better to use PV = nRT to solve for n in terms of the initial volume (lets call it V_o).  Then, once you find n, plug that value into the equation using the second set of numbers to find the temperature.

[Bronx19]

How do I find the initial volume using PV = nRT if only pressure and temperature are known, would I not need to know the moles of gas?

[Yggdrasil]

You know that n is constant so n₁ = n₂.  Now, you are not given any volumes but you do know that V₂ = 1.028V₁ (i.e. V₂ is 2.8% greater than V₁).  Now, you can take two approaches:

1) Choose an arbitrary value for V.  Likely, if you can't figure out the initial volume, the initial volume doesn't matter, and you can solve the problem using any value for the initial volume.  For example, let V₁ = 1L so that V₂ =  1.028L, then use those values to find T₂.  Just to make sure that the value of the initial volume doesn't matter, take V₁ to be a different value (say 2L) and see if you still get the same result for T₂.

2) Leave V₁ as a variable.  When you calculate T₂ it should cancel out.  (This is the mathematically rigorous way of showing that the value of V₁ doesn't matter.)

[Bronx19]

Ah, now its clearer.

(P1V1)/T1 = (P2V2)/T2

(206KPa x 1.028L) / 0.634 = 334.23K or 61.2C.

Hopefully Im correct?

[Yggdrasil]

Looks good to me.