Chemistry - Acids, bases and buffers question help! Watch

science_geeks
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Hi everyone,

I'm stuck on this question, I've tried a few different methods but I can't get to the mark scheme answer. Please could someone help me

Here is the question:

This question involves calculations about two strong acids and one weak acid. All measurements were carried out at 25 °C.
3 –3
(a) A 25.0 cm sample of 0.0850 mol dm hydrochloric acid was placed in a beaker and 100 cm3 of distilled water were added. Calculate the pH of the new solution formed. Give your answer to 2 decimal places.

Thank you in advance
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Mermaidqueen
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Is the pH 3.37? I've tried a method but I'm not sure if its right
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science_geeks
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(Original post by Mermaidqueen)
Is the pH 3.37? I've tried a method but I'm not sure if its right
Thanks for your reply, but the mark scheme says the pH should be 1.77
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cciaran
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Really ugly late night working out but hope this helps ! If you have any questions feel free to ask
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Mermaidqueen
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ohhhhh, that's how you do it! I just have one teeny tiny question, if we add 100cm^2 of water, isn't there a dilution factor of 5 how come the number of moles of HCl doesn't change?
(Original post by cciaran)
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Really ugly late night working out but hope this helps ! If you have any questions feel free to ask
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science_geeks
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(Original post by cciaran)
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Really ugly late night working out but hope this helps ! If you have any questions feel free to ask
Thank you! That makes complete sense I completely forgot to work out the new volume and therefore new concentration! PS, your workings are quite neat!
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cciaran
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As far as I know the number of moles doesn’t change if the volume increases, the moles are still in there, just in a larger volume. The concentration only changes which is what you had to work out.

Say you have 5g (imagine it as moles) of sugar in 100cm3 of water, you have 5 moles of sugar solution.
If you increase the volume by adding 900cm3, you still have 5g, or 5 moles, just in a larger volume
(Original post by Mermaidqueen)
ohhhhh, that's how you do it! I just have one teeny tiny question, if we add 100cm^2 of water, isn't there a dilution factor of 5 how come the number of moles of HCl doesn't change?
Last edited by cciaran; 1 month ago
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Mermaidqueen
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Oh I see... That makes so much sense! Thank you, I completely forgot that the moles is number of atoms so obviously it shouldn't change 🤦🏻*♀️🤦🏻*♀️
(Original post by cciaran)
As far as I know the number of moles doesn’t change if the volume increases, the moles are still in there, just in a larger volume. The concentration only changes which is what you had to work out.

Say you have 5g (imagine it as moles) of sugar in 100cm3 of water, you have 5 moles of sugar.
If you increase the volume by adding 900cm3, you still have 5g/moles, but in a larger volume
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cciaran
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No problem, I’m glad it makes sense! Thank you
(Original post by science_geeks)
Thank you! That makes complete sense I completely forgot to work out the new volume and therefore new concentration! PS, your workings are quite neat!
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andrea.lab
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an easy way is:
original H+ conc x old volume/new volume = new H= conc. then you can plug this into the orginal -logH+ to find the pH.
of course this is only for monobasic acids and units for both volumes have to be the same. and you can also use this for all the acid and base dilution questions. i got 1.77? is that correct





(Original post by science_geeks)
Hi everyone,

I'm stuck on this question, I've tried a few different methods but I can't get to the mark scheme answer. Please could someone help me

Here is the question:

This question involves calculations about two strong acids and one weak acid. All measurements were carried out at 25 °C.
3 –3
(a) A 25.0 cm sample of 0.0850 mol dm hydrochloric acid was placed in a beaker and 100 cm3 of distilled water were added. Calculate the pH of the new solution formed. Give your answer to 2 decimal places.

Thank you in advance
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science_geeks
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(Original post by andrea.lab)
an easy way is:
original H+ conc x old volume/new volume = new H= conc. then you can plug this into the orginal -logH+ to find the pH.
of course this is only for monobasic acids and units for both volumes have to be the same. and you can also use this for all the acid and base dilution questions. i got 1.77? is that correct




That’s an easy way too! Yep, you’re right 1.77 is the pH 😊
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science_geeks
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(Original post by cciaran)
As far as I know the number of moles doesn’t change if the volume increases, the moles are still in there, just in a larger volume. The concentration only changes which is what you had to work out.

Say you have 5g (imagine it as moles) of sugar in 100cm3 of water, you have 5 moles of sugar solution.
If you increase the volume by adding 900cm3, you still have 5g, or 5 moles, just in a larger volume
Hi again! cciaran, I needed some help with a maths question and was wondering whether you'd be able to help? Thank you!
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cciaran
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Sorry I just saw this, but if you still need help I can try!
(Original post by science_geeks)
Hi again! cciaran, I needed some help with a maths question and was wondering whether you'd be able to help? Thank you!
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science_geeks
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(Original post by cciaran)
Sorry I just saw this, but if you still need help I can try!
Thank you! Sorry, I know it's not chemistry but here it is: Q5d. Thank you
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