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# Chem help Watch

1. A 250cm^3 solution of NaOH was prepared. 25.0cm^3 of this solution required 28.2cm^3 of 0.100moldm^3 HCl for neutralisation. Calculate what mass of NaOH was dissolved to make up the original 250cm^3 of solution.?? Please I need help with this.
2. (Original post by fatmaz00)
A 250cm^3 solution of NaOH was prepared. 25.0cm^3 of this solution required 28.2cm^3 of 0.100moldm^3 HCl for neutralisation. Calculate what mass of NaOH was dissolved to make up the original 250cm^3 of solution.?? Please I need help with this.
What have you done so far?
3. (Original post by fatmaz00)
A 250cm^3 solution of NaOH was prepared. 25.0cm^3 of this solution required 28.2cm^3 of 0.100moldm^3 HCl for neutralisation. Calculate what mass of NaOH was dissolved to make up the original 250cm^3 of solution.?? Please I need help with this.
1. Find the number of moles of a HCl in the question.

2. Write out a full equation.

3. Use molar ratios to obtain the correct number of moles of NaOH.

4. Multiply appropiatley for the 250cm3 volume

5. Use m = n x mr to obtain correct mass.
4. (Original post by GradeA*UnderA)
N of HCl = 0.0282dm3 x 0.1 = 0.00282

NaOH + HCl -> NaCl + H2O

Molar ratio of NaOH and HCl is 1:1

So N of NaOH = 0.00282 moles in 25cm3

In 250cm3 we multiply the number of moles by 10 since its 10 times bigger volume

So N of 250cm3 NaOH = 0.0282 moles

Mass of NaOH dissolved to make original solution:
Mass = 0.0282 x 40 = 1.128g

1.13g (3sf)
Do not write full solutions, delete this post or most of it now
5. (Original post by richpanda)
Do not write full solutions, delete this post or most of it now
It's from an OCR past paper on their website. What's the fuss?
6. (Original post by GradeA*UnderA)
It's from an OCR past paper on their website. What's the fuss?
Forum rules say don't post full solutions to members, especially if they have made no effort themselves.
7. (Original post by richpanda)
Forum rules say don't post full solutions to members, especially if they have made no effort themselves.
*sigh* fine.
8. Sorry and thank you for the help, I had already calculated the solutions for HCl. I just did not know what to do from there on. Next time I will remember.
9. (Original post by GradeA*UnderA)
1. Find the number of moles of a HCl in the question.

2. Write out a full equation.

3. Use molar ratios to obtain the correct number of moles of NaOH.

4. Multiply appropiatley for the 250cm3 volume

5. Use m = n x mr to obtain correct mass.
(Original post by richpanda)
Do not write full solutions, delete this post or most of it now
The OP can still see the full worked solution in the quote, Panda.

You fail
10. (Original post by AngryRedhead)
The OP can still see the full worked solution in the quote, Panda.

You fail
lol. the point is that GradeA*UnderA doesn't do it again.
11. (Original post by richpanda)
lol. the point is that GradeA*UnderA doesn't do it again.
Where exactly in the forum rules does it say you can't do that? I've been answering questions on TSR for ages and I've never come across that

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