Titration Calculations

1. Jun 2, 2009

ajassat

1. The problem statement, all variables and given/known data
2.0g of pure undiluted HsSO4 is cautiously added to water and the solution diluted to 500 cm3. What volume of 0.1M potassium hydroxide, KOH, would exactly neutralise 25cm3 of the acid solution?

2. Relevant equations
concentration = number of moles/volume
mass in grams = number of moles*relative formula mass

3. The attempt at a solution

H2SO4
2.0g/500cm3
Divide by 20 to get values for 25cm3
0.1g/25cm3

Conversion of grams to M
mass in grams = number of moles*relative formula mass
0.1g = number of moles*98
0.1g/98 = number of moles = 1.02moles

We now have

1.02M/25cm3

Let us work out the concentration...

25/100 * 1.02 = 0.0255molar

Now let us work out the volume

Concentration = number of moles/volume
Concentration*volume = number of moles
volume = number of moles/concentration
volume = 1.02/0.0255
volume = 40cm3

Is this correct?
1. The problem statement, all variables and given/known data

2. Relevant equations

3. The attempt at a solution

2. Jun 2, 2009

Bohrok

After you calculate the moles of H2SO4, I think you have problems. 1.02M/25cm3 makes no sense with molarity over volume, but it should be 1.02 moles/25 cm3. You also don't use the molarity of the KOH.
Write a balanced equation first.

You should start this way, which has the moles you calculated.

$$25 cm^3 solution \left(\frac{1.02 mol H_2SO_4} {500 cm^3 solution}\right)$$

You have moles of the acid, then multiply it by conversion factors using the balanced equation and the molarity of the base, and you will get the volume of the base.

3. Jun 2, 2009

ajassat

Can you show me the complete process?

4. Jun 2, 2009

Bohrok

Write the balanced equation first.

5. Jun 2, 2009

ajassat

2koh + h2so4 --> k2so4 + 2h2o

6. Jun 2, 2009

ajassat

It is a 2:1 ratio.

7. Jun 2, 2009

ajassat

If H2SO4 is 1.02 then KOH is 2.04moles

8. Jun 2, 2009

ajassat

volume = concentration/number of moles
volume = 0.1M/2.04 (KOH) * 1000
volume = 49?

9. Jun 2, 2009

Bohrok

So you know you need twice as much KOH as H2SO4 given from
$$25 cm^3 solution \left(\frac{1.02 mol H_2SO_4} {500 cm^3 solution}\right)$$

which gives you moles of KOH. The molarity of the base is 0.1M KOH or $$\frac {0.1 mol KOH} {1 L KOH}$$. How can you use this fraction with your moles of KOH from above to give you the volume of KOH solution?

Edit: Looks like you don't need these steps. Read the next post.

10. Jun 2, 2009

Bohrok

1.02 is the moles of H2SO4 in the 500 cm3 of acid solution, but you need to know how many moles are in 25 cm3 of solution since that is how much is being neutralized by the base. You need to calculate this from what I wrote above. That will give you the actual number of moles of acid you're working with in 25 cm3 that's being neutralized. Then the 2:1 ratio you said is correct and you should get the correct answer.

11. Jun 2, 2009

ajassat

0.051 moles in 25cm^3. Therefore 0.102 moles for KOH.

Revised volume calculation

V = 0.1/0.102

12. Jun 2, 2009

Bohrok

Yes, that looks right.

13. Jun 2, 2009

ajassat

I need to multiply the answer by 1000 right?

14. Jun 2, 2009

Bohrok

That would give you mL of solution, but the question didn't say in what units to give the volume. You should be okay with leaving it in L unless you're expected to use a certain unit when the question doesn't specify.

15. Jun 2, 2009

Staff: Mentor

16. Jun 2, 2009

Bohrok

You do have a mistake here. 0.1/98 is .00102, so that is the actual number of moles of H2SO4. This is also wrong

Concentration = moles/volume, so volume = moles/concentration

I hadn't checked your work thoroughly before, but I think these are all the mistakes you had.