chemistry question - got me stumped.

[johnfm]

The following question was set today in my daughter's first chemistry class today.

Question is as follows:

Dissolve 1 g of potassium manganate in 1000 cm3 of water. Then take 100 cm3 of this dilution and add 900 cm3 to dilute it to 1000 cm3.

Now take 100 cm3 of this once-dilute solution and dilte it to 1000cm3. repat until you have diluted it 6 times. This dilution is equivalent to the original 1g of potassium manganate in 1000 000 000 cm3 of water.


Suppose that 1g of potassium manganate has a volume of 1 cm3 and that there is one particle of potassium manganate in every drop of the final 1000 000 000 cm3 solution of the faint pink solution. If there are 20 drops in 1cm3 of this faint pink solution, how many particles of potassium manganate are there in 1000 000 000 cm3 of the faint pink solution.

Now calculate the VOLUME of one particle of potassium manganate.

Good luck!

[loftylad]

4cm3

HTH

[Simpo Two]

Is 'particle' the new word for molecule?

A molecule will have the same volume regardless of how much water it's in.

Edited by Simpo Two on Friday 12th September 21:54

[Pesty]

Opens thread with my grade A at O-level.

Reads question eyes go funny, backs out of thread

[jimmyjimjim]

Poorly worded, or is it just me - the first half of the question appears to give the answer?
The second part seems to be 1/1,000,000,000 cm3?

[johnfm]

Tell me about it? They dumb down the question by using particle instead of molecule, but then set a question which, frankly, no 12 year old should have to answer on the third day of school!

I get 5 x 10-11 cm3, by the way.

Edited by johnfm on Friday 12th September 22:30

[johnfm]

I think I may need to speak with my dfaughter's chemistry teacher. If she set the question, what hope is there!??

[Tunku]

Why have a Maths question in Chemistry?

[johnfm]

exactly what I thought. I tried to console my daughter - it's just a simple maths question - or so I thought.

[Muntu]

johnfm said:

This dilution is equivalent to the original 1g of potassium manganate in 1000 000 000 cm3 of water.

Oh no it isn't.

Can't be arsed to work out the answer to the question though, it seems to have been written by somebody half way through an ESOL course.

[Rebuker]

I dunno.

But do mention Le Chatelier. They spunk themselves over that

[Simpo Two]

Rebuker said:

But do mention Le Chatelier. They spunk themselves over that

Not Avogadro?

[chris_w666]

That got me stumped totally.

I'm not sure that question even makes sense, its more a comprehension test than a maths or science question.

[johnfm]

Muntu said:

johnfm said:

This dilution is equivalent to the original 1g of potassium manganate in 1000 000 000 cm3 of water.

Oh no it isn't.

Can't be arsed to work out the answer to the question though, it seems to have been written by somebody half way through an ESOL course.

Well, the first step is 1g in 1000cm3. Then you take 100cm3 of this, you have 0.1g of potassium manganate. Diluted to 1000cm3 is now 0.1g in 1000cm3. Repeat, leads to 0.001g in 1000cm. So, repeating 6 times give 0.000001g in 1000cm3 (i.e. 1 x 10-6g in 1000 cm3. So, it is equivalent dilution to 1 g in 1,000 x 10[small]6{/small] cm3.

It is a terribly worded question - hence my annoyance at a chemistry teacher at a £10k/year school

[Simpo Two]

johnfm said:

It is a terribly worded question - hence my annoyance at a chemistry teacher at a £10k/year school

From the question I assumed it was some illiterate PC comprehensive. But for £10K pa I'd march in and ask them what the fk is wrong with 'molecules'? The kids are going to have to learn scary words sooner or later; may as well get it over with.

NB: Who took the per off the manganate?

[Schmeeky]

Well, I did chemistry at A-level, but that's no use at all here..... it's a totally crap question that asks a fairly simple thing in an unnecessarily complex way... but it took loads of figuring out to work out what the question actually was!! I get 0.00002cm^3

Gimmie a minute and I'll try to show my working.... brb...

ETA Please bare in mind I've been at the pub most of the afternoon, but going back to the question it seems to make even less sense now.... Foisting a question like this on a kid on their third day at school is child abuse, quite frankly....

Edited again to add - I may be more than a minute... my head has just exploded... But if it's of any interest, there were 2.7x10^23 molecules, the mess covered 0.75m^2 on the floor, and it took 0.417seconds to hit the floor. The question is - how many neutrons are in my left toenail??

Edited by Schmeeky on Friday 12th September 22:40

[quyen]

johnfm said:

If there are 20 drops in 1cm3 of this faint pink solution

20 drops of what?

[johnfm]

my working is as follows:

1g in 1000cm3

diluted 6 times gives 1g in 1000 x 10^6 cm3

there are 20 drops per cm3, so 20 x 10^9 drops in the 1000 x 10^6cm3 of solution

so, if there is one particle per drop, there is 20 x 10^9 'particles' in the solution.

so, these 20 x 10^9 particles weigh 1g, therefore 1 particle weighs:

1/20x10^9.

This is 5 x 10^-11 cm3.

[wiffmaster]

Tunku said:

Why have a Maths question in Chemistry?

There speaks a man who hasn't had the pleasure of the "Edexcel A-Level Chemistry Experience". It's all maths and remembering colours. The amount of actual chemistry learnt in the two-year course could be written on the back of a postage stamp.

Oh, and for the record: Both of my chemistry teachers were unable to write questions or explanations in coherent, logical English. Brilliant minds, but absolutely terrible at expressing themselves. A trend, perhaps?

Edited by wiffmaster on Friday 12th September 22:33

[johnfm]

it is totally ste isn't it.

This chemistry teacher will be unhappy to see me at parent's evening!