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VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: lacoste on March 15, 2009, 11:21:18 am

Title: Standard solution and Primary Standard
Post by: lacoste on March 15, 2009, 11:21:18 am: I can list the properties of each of them.

But could someone please define what it means/is? (sort of hard when the properties of those must be usd)

another q, where are each of them used in an experiment?

thanks
Title: Re: Standard solution and Primary Standard
Post by: jackinthepatch on March 15, 2009, 11:26:51 am: Primary standard is a substance that is so pure that it's number of mole can be calculated very accurately from it's mass.

A standard solution is simply a solution of known concentration.

You use a primary standard to make up a standard solution, and this standard solution is then used in an experiment to determine the concentration/mass of a known substance in solution. This can be determined by titrating the standard solution with the solution under investigation.
Title: Re: Standard solution and Primary Standard
Post by: pHysiX on March 15, 2009, 11:57:24 am: A primary standard must also be quite readily available and affordable to obtain. It must also not react with the atmosphere.
Title: Re: Standard solution and Primary Standard
Post by: TrueTears on March 15, 2009, 12:00:38 pm: Primary standard should also should have a known formula, have a high molar mass and be ready to obtain in pure form.

examples of primary standards include:

bases: $Na_2CO_3$

acids: $KH(C_8H_4O_4)$
Title: Re: Standard solution and Primary Standard
Post by: lacoste on March 15, 2009, 12:16:28 pm: ahhh thanks!!!!!!!!!!!!!!!!!!!!!!!!

just another q: how do I determine which substance is in excess or is the limiting reactants?

do the coeffiecients in front of the substance affect anything?
Title: Re: Standard solution and Primary Standard
Post by: TrueTears on March 15, 2009, 12:17:41 pm: Quote from: lacoste on March 15, 2009, 12:16:28 pm
ahhh thanks!!!!!!!!!!!!!!!!!!!!!!!!

just another q: how do I determine which substance is in excess or is the limiting reactants?

do the coeffiecients in front of the substance affect anything?
Yes they do.

Basically you have to work out the mole of each substance and them use the coefficients as the ratio to work out which is limiting and which is in excess
Title: Re: Standard solution and Primary Standard
Post by: lacoste on March 15, 2009, 12:19:16 pm: So if the substance with the lowest mole ratio is the limiting??????????????
Title: Re: Standard solution and Primary Standard
Post by: TrueTears on March 15, 2009, 12:20:42 pm: Quote from: lacoste on March 15, 2009, 12:19:16 pm
So if the substance with the lowest mole ratio is the limiting??????????????

no, not always, it depends on the question
Title: Re: Standard solution and Primary Standard
Post by: jackinthepatch on March 15, 2009, 12:22:47 pm: Write the equation out. Then calculate the number of moles you have of each of the reactants. The coefficients tell you how many moles one substance requires to react with it.

Eg.
1HCl + 1NaOH --> 1H₂O + 1NaCl

This tells you that one mole of HCl reacts exactly with one mole of NaOH.

By calculating the number of mole of each of the reactants you have available, you can determine which reactant is in excess.
Say you had 36.5g of HCl and 42g of NaOH. This would mean you would have 1 mole of HCl and 1.05 mole of NaOH, meaning NaOH is in excess because HCl only requires 1 mole of NaOH to react with, and you would be supplying 1.05 mole of NaOH. Therefore HCl is limiting.

You base your following calculations on the limiting reactant.
Title: Re: Standard solution and Primary Standard
Post by: lacoste on March 15, 2009, 12:59:41 pm: Quote from: TrueTears on March 15, 2009, 12:20:42 pm
Quote from: lacoste on March 15, 2009, 12:19:16 pm
So if the substance with the lowest mole ratio is the limiting??????????????

no, not always, it depends on the question

What do you mean by depend on the question?

thanks truetears!!

Thanks jackinthepatch for the clear up
Title: Re: Standard solution and Primary Standard
Post by: TrueTears on March 15, 2009, 01:00:47 pm: Well every question would have different equations and therefore different coefficients etc

Basically Jack's example tells you everything you need to do in an excess/limiting question.
Title: Re: Standard solution and Primary Standard
Post by: jackinthepatch on March 15, 2009, 01:01:54 pm: Yeah...sorry if my example was worded a bit wierdly, I rushed it so I hope it makes sense haha.
Title: Re: Standard solution and Primary Standard
Post by: lacoste on March 15, 2009, 01:02:05 pm: no worries, jackinthepatch, its worded great!

understand it, but

is the lowest mole when divided by the coefficient the limiting reactant?

always, no tricks or anything