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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: DMOC on October 25, 2009, 01:50:59 PM

Title: Why isn't it important to record the exact amount of water used?
Post by: DMOC on October 25, 2009, 01:50:59 PM

Question: A student dossolves 0.25g of KHPh in 0.050 L of water while performing an acid and base titration to determine the [ ] of a KOH solution. The student washed down the sides of the flask with 5 mL of deionized water, but did nto include that volume in the total volume. Why is it not necessary to record the exact amount of water used to make the solution?

Here is my answer, which my teacher told me was wrong somewhere.

It is not important because al that matters is the measure on the buret - where the titrant starts and where it ends. For instance, when I did this lab before, I took the difference between the two measurements to figure out how much titrant I used. What the student should do is not add any more titrant in the buret which would alter the measurement.

Title: Re: Why isn't it important to record the exact amount of water used?
Post by: nj_bartel on October 25, 2009, 01:55:24 PM

I think you'd need to take that 5 mL into account...

Title: Re: Why isn't it important to record the exact amount of water used?
Post by: Arctic-Nation on October 25, 2009, 02:58:45 PM

I've got to disagree with nj_bartel. The exact amount of water is not important, only the exact amount of KHPh, which is the only thing you need to calculate the exact concentration of the KOH solution with. DMOC's answer isn't wrong per se, but a bit limited and poorly formulated.

Title: Re: Why isn't it important to record the exact amount of water used?
Post by: Borek on October 25, 2009, 03:51:04 PM

I would say DMOC answer doesn't address the real issue.

Title: Re: Why isn't it important to record the exact amount of water used?
Post by: nj_bartel on October 25, 2009, 03:51:13 PM

Ok, I misunderstood the experiment - agree with arctic.  Was thinking you were making titrant solution with the KHP.

Title: Re: Why isn't it important to record the exact amount of water used?
Post by: DMOC on October 25, 2009, 07:48:52 PM

Ok, so how about this new answer?

It is not important to record the exact amount of water used because what is important is the exact amount of KHPh. That is the only thing I need to calculate the exact concentration of the KOH solution.