VCAA 2009 Q10b

[H2O]

The solutions on finding out m(Ba) added to vol flask don't make sense to me. 
Where do you pull out the numbers from?
Thanks guys

[soccerboi]

We want to find the mol of Ba2+ that reacted in the 10ml volumetric flask.

To do this we need to calculate the initial mass in this 10 ml flask.
m(Ba2+)= 200mg but this is in 1L(1000ml), since we have only 2ml we need to adjust this by multiplying 200mg by (2/1000).
We get m(Ba2+)= 200mg X (2/1000) = 0.400 mg in 10ml volumetric flask(amount initially)

now we figure the mass of Ba2+ that has reacted.
From the graph,and from part a, we figured that the m(Ba2+) remaining or in other words, is in excess, is 0.19mg.

So to figure the mass of Ba2+ that reacted to produce the precipitate, we simply subtract mass excess from mass of initial.
mass(Ba2+)reacted= initial-excess=0.400-0.19=0.21mg

now to find mol of Ba2+ reacted.
n(Ba2+)= (0.21X10^-3)/137.3=1.53X10^-6mol (2 decimal places because of the subtraction in the above calculation)
Hope that helps 

Edit: just realised you were just asking for the m of Ba2+ added to volumetric flask...so yeh just refer to bolded bit above

[soccerboi]

With this question(VCAA 2009 Q10) ,can someone clarify this:
From 100 ml ---> 25 ml ---> 10 ml

From 100 ml to 25 ml, mol changes but concentration stays the same.
From 25 ml to 10 ml, mol stays the same concentration chnages.

With dilutions and transference of liquids will it always be one changes and one stays the same? Can there ever be cases where both mol and concentration changes or both stays the same?

[aznxD]

If there is a dilution, the concentration changes, but the amount stays the same.
If your taking an aliquot, the amount changes, but the concentration stays the same.

[charmanderp]

With this question(VCAA 2009 Q10) ,can someone clarify this:
From 100 ml ---> 25 ml ---> 10 ml

From 100 ml to 25 ml, mol changes but concentration stays the same.
From 25 ml to 10 ml, mol stays the same concentration chnages.

With dilutions and transference of liquids will it always be one changes and one stays the same? Can there ever be cases where both mol and concentration changes or both stays the same?

I believe there was a question in 2008 where instead of diluting hydrochloric acid with water the subject of the question accidentially 'diluted' it with sodium hydroxide and hence both amount and concentration changed.

[soccerboi]

Ok thanks guys, another question with this exam, Q7 a
How can you deduce which has undergone oxidation or reduction based from the diagram given?
Or do we have to memorise which is which?

[charmanderp]

Oxidation numbers? Also glucose is a reducing sugar but I can't remember if that's relevant to that question.

[soccerboi]

Oxidation numbers? Also glucose is a reducing sugar but I can't remember if that's relevant to that question.

Yeh i used oxidation numbers, but thought there was other ways, because cellulose was in the diagram and i didn't know its molecular formula.