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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: steph_r on September 08, 2008, 06:40:23 AM

Title: primary standard :D
Post by: steph_r on September 08, 2008, 06:40:23 AM: Hello all,
I was just wondering..

Why is it a waste of time to weigh out accurately a primary standard of doubtful purity?

Is it perhaps becasue it has the property of absorbing?
Can someone please explain?

Thank you in advance :D
Title: Re: primary standard :D
Post by: Borek on September 08, 2008, 07:09:16 AM: What do you weight the substance for?
Title: Re: primary standard :D
Post by: steph_r on September 08, 2008, 07:18:15 AM: I was doing volumetric analysis at school. We did the analysis to determine the concentration of a hydrochloric acid solutution.

I hope this helps :D
Title: Re: primary standard :D
Post by: Borek on September 08, 2008, 08:09:32 AM: I am not asking about this particular analysis, I am asking about the general idea behind the experiment.
Title: Re: primary standard :D
Post by: steph_r on September 09, 2008, 04:22:39 AM: To work out concentration?

Not sure
Title: Re: primary standard :D
Post by: Borek on September 09, 2008, 04:45:23 AM: Do you understand how the analysis is done? It is based on the stoichiometry of the reaction. Substances react in a stoichiometric ratio given by the reaction equation. If you know exact amount of one reactant, you can calculate exact amount of the other.
Title: Re: primary standard :D
Post by: steph_r on September 09, 2008, 06:01:13 AM: Sorry, im still unsure then why it is pointless to weigh out accuraetly an impure primary standard. Could you please explain?

Thank you for your patience
Title: Re: primary standard :D
Post by: Borek on September 09, 2008, 06:05:02 AM: Will you know exact amount of the reactant?
Title: Re: primary standard :D
Post by: steph_r on September 09, 2008, 06:39:00 AM: Yes i will know the exact amount, after weighing or calculating
Title: Re: primary standard :D
Post by: steph_r on September 09, 2008, 07:32:56 AM: Is it because there are other impurities mixed in and therefore, makes it harder to weigh the desired substance??
Title: Re: primary standard :D
Post by: sjb on September 09, 2008, 07:43:29 AM: Quote from: steph_r on September 09, 2008, 06:39:00 AM
Yes i will know the exact amount, after weighing or calculating

If I weigh out 1.0000 g of a compound of unknown purity, how much of the active compound do I have?

What does Primary standard mean?

S
Title: Re: primary standard :D
Post by: Borek on September 09, 2008, 08:32:01 AM: Quote from: steph_r on September 09, 2008, 07:32:56 AM
Is it because there are other impurities mixed in and therefore, makes it harder to weigh the desired substance??

Imagine you have kitchen salt mixed with sugar. You don't know composition of the mixture. You have exactly 1.0000 kg of the mixture. Do you know how much kitchen salt do you have?
Title: Re: primary standard :D
Post by: steph_r on September 10, 2008, 06:06:08 AM: no we dont know, because salt and sugar are mixed together? is that correct?
Title: Re: primary standard :D
Post by: Borek on September 10, 2008, 06:58:32 AM: Quote from: steph_r on September 10, 2008, 06:06:08 AM
no we dont know, because salt and sugar are mixed together? is that correct?

Will you know amount of salt if the mixture is exactly 75% salt and 25% sugar?