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Chemistry AS Help- Volumetric Calculations

I dont know how to even start his equation, someone please explain to me step by step and i should be able to get it

In this series of calculations you should start by writing the equation for the reaction taking place then generate the molarity/volume ratio. In some cases you will need to calculate the molarity of the solutions before you start the main part of the question.

For questions 1-10 calculate the molarity of the first named solution from the data below....

25cm^3 of sodium hydroxide reacts with 21.0cm^3 of 0.2 mol dm^-3 HCL

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itsmyname
25cm^3 of sodium hydroxide reacts with 21.0cm^3 of 0.2 mol dm^-3 HCL


NaOH + HCl --> H2O + NaCl

moles of HCl = 21/1000 x 0.2 = 0.0042 moles
moles of NaOH = 0.0042 => concentration = 0.0042/(25/1000) = 0.168 mol dm^-3
Reply 2
Wait wait im sorry youve lost me. Can u please explain step by step?
okay, firstly we know the chemical equation (above) so we know that 1 molecule of NaOH will react with 1 molecule of HCl

Using the formula concentration = moles/volume we can work out the number of moles of HCl in the question. Volume = 21cm^3 = 0.021dm^3 (1 cm^3 = 0.001 dm^3) and concentration is 0.2 mol dm^3 so rearranging the formula; moles = concentration x volume = 0.2 x 0.021 = 0.0042

so we know the moles of HCl and NaOh are equal - moles of NaOH = 0.0042 also

use the formula: concentration = moles/volume = 0.0042mol dm^3/(25cm^3/1000) = 0.168 mol dm^3
Reply 4
But how did u work out that that is the formula? Ie. how do you u know NaOH + HCl makes H20 and NaCl?
itsmyname
But how did u work out that that is the formula? Ie. how do you u know NaOH + HCl makes H20 and NaCl?


acid + base ---> water + salt

so essentially H+ + OH- ---> H2O

and HCl ---> H+ + Cl- => source of H+
NaOH ---> Na+ + OH- => source of OH-
Reply 6
EierVonSatan
NaOH + HCl --> H2O + NaCl

moles of HCl = 21/1000 x 0.2 = 0.0042 moles
moles of NaOH = 0.0042 => concentration = 0.0042/(25/1000) = 0.168 mol dm^-3


Ok so I get the bold but Im not getting the second part.
itsmyname
Ok so I get the bold but Im not getting the second part.


we know theres 0.0042 moles of NaOH (from the chemical equation)

concentration = moles/volume...we know the moles and the volume and so can work out the concentration
Reply 8
But isnt 0.0042 moles the number of moles for HCl? Not the moles for NaOH?
itsmyname
But isnt 0.0042 moles the number of moles for HCl? Not the moles for NaOH?


There are equal amounts (moles) of both because;

1HCl + 1NaOH ---> 1H2O + 1NaCl

one must react with one
Reply 10
Oh ok... I think I get it. Is that the ratio part?

So if the ratio was ( i know it wont be but for arguments sake), HCl + 2NaOH would u multiply 0.0042 by 2?
itsmyname
Oh ok... I think I get it. Is that the ratio part?

So if the ratio was ( i know it wont be but for arguments sake), HCl + 2NaOH would u multiply 0.0042 by 2?


yes! :biggrin:
Reply 12
Ahhh, I think I am Getting it!! Something my bitchy chemistry teacher couldnt do!

But the original question is asking for the Molarity of the first named solution, so therefore I should just leave it at 0.0042 instead of working out the concentration, right?
itsmyname
Ahhh, I think I am Getting it!! Something my bitchy chemistry teacher couldnt do!

But the original question is asking for the Molarity of the first named solution, so therefore I should just leave it at 0.0042 instead of working out the concentration, right?


you're given the molarity (molarity means concentration)
Reply 14
EierVonSatan
you're given the molarity (molarity means concentration)


Huh? What? I think thats the molarity of the HCl and the questions asking for the molarity of the first part Ie Sodium Hydroxide (NaOH)
itsmyname
Huh? What? I think thats the molarity of the HCl and the questions asking for the molarity of the first part Ie Sodium Hydroxide (NaOH)


I mean your given the molarity of the HCl solution and you have to work out the molarity (concentration) of the NaOH solution...
Reply 16
Ah, oook. Yes Ive got it

Thanks a lot for the help, really appreciate it :biggrin:
U have potentially saved me from being kicked out of the course

I'll let u know u if I get stuck further on :biggrin:
Reply 17
Ok Now im stuck on another question it asks;

20cm^3 of hydrochloric acid reacts with 20Cm^3 if a solution of NaOH containing 40g dm^-3 of NaO
itsmyname
Ok Now im stuck on another question it asks;

20cm^3 of hydrochloric acid reacts with 20Cm^3 if a solution of NaOH containing 40g dm^-3 of NaO


I assume you mean NaOH and not NaO :p:
use the equation concentration (g dm^3) = concentration (mol dm^3) x Mr (gmol^-1)
Reply 19
is the molarity/volume ratio the concentration?:s-smilie::s-smilie: