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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Reply 1

EierVonSatan

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

itsmyname

Wait wait im sorry youve lost me. Can u please explain step by step?

Reply 3

EierVonSatan

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

itsmyname

But how did u work out that that is the formula? Ie. how do you u know NaOH + HCl makes H20 and NaCl?

Reply 5

EierVonSatan

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

itsmyname

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.

Reply 7

EierVonSatan

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

itsmyname

But isnt 0.0042 moles the number of moles for HCl? Not the moles for NaOH?

Reply 9

EierVonSatan

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

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?

Reply 11

EierVonSatan

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!

Reply 12

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?