Chemo P5 tips

[angelgirl:)]

When preparing a solution of a fixed concentration from a given parent solution of concentration , say, 2.0 mol/dm3​

​

A. We are required to prepare a solution of concentration of 1.0 mol/dm3 with volume 250 cm3 ( the concn cannot be greater than 2 !!!!!!!!!!!!!!)​

a. Add 100 cm3 of the parent solution to a volumetric flask of marking at 250 cm3​

b. Top off with water to the mark of 250 cm3. Use a wash bottle for this purpose to have greater control over the addition process​

​

When required to prepare 250 cm3 of 0.5 mol/dm3 solution of a crystal of Mr 50g​

​

First realize that we need only 250 cm3, not 1 dm3​

Now in 250 cm3 there will be: 50/4 = 12.5 g of the solid​

​

So, first add 12.5 g of solid to 50 cm3 of water in a BEAKER ( not the volumetric flask yet). Also note that the volume of water is less than 250 cm3.​

Stir properly and if the solid doesn't dissolve add more water until it fully dissolves​

Then transfer the solution from the beaker to a volumetric flask. Remember to rinse the beaker with water and transfer the solution to the volumetric flask​

Stopper the flask and shake properly​

Finally, add the required volume of water to make the solution upto 250 cm3 USE A FUNNEL OR YOU RISK OVERSHOOTING THE MARK​

1dm3 = 1000 cm3
= 50g
but we r making in 250cm3
which is 1000/250=4
so 50/4=12.5g...
hope u got it..

The first calculation is incorrect as someone pointed out already. The second calculation is also incorrect.
moles required in the 250cm3 solution = .250 x .5 = 0.125
mass required = moles x Mr = .125 x 50 = 6.25g (not 12.5g)

hmmm... both make sense....but how come we divide Mr by 4..this makes it little impracticle..isnt it?

 

[angelgirl:)]

When preparing a solution of a fixed concentration from a given parent solution of concentration , say, 2.0 mol/dm3​

​

A. We are required to prepare a solution of concentration of 1.0 mol/dm3 with volume 250 cm3 ( the concn cannot be greater than 2 !!!!!!!!!!!!!!)​

a. Add 100 cm3 of the parent solution to a volumetric flask of marking at 250 cm3​

b. Top off with water to the mark of 250 cm3. Use a wash bottle for this purpose to have greater control over the addition process​

​

When required to prepare 250 cm3 of 0.5 mol/dm3 solution of a crystal of Mr 50g​

​

First realize that we need only 250 cm3, not 1 dm3​

Now in 250 cm3 there will be: 50/4 = 12.5 g of the solid​

​

So, first add 12.5 g of solid to 50 cm3 of water in a BEAKER ( not the volumetric flask yet). Also note that the volume of water is less than 250 cm3.​

Stir properly and if the solid doesn't dissolve add more water until it fully dissolves​

Then transfer the solution from the beaker to a volumetric flask. Remember to rinse the beaker with water and transfer the solution to the volumetric flask​

Stopper the flask and shake properly​

Finally, add the required volume of water to make the solution upto 250 cm3 USE A FUNNEL OR YOU RISK OVERSHOOTING THE MARK​

hmmm... both make sense....but how come we divide Mr by 4..this makes it little impracticle..isnt it?

even i go with the second method...

 

[histephenson007]

How to prepare crystals (which is also linked with solubility)
Many methods are mentioned in MS but I found this to be the simplest:

How to measure solubility:
6. Measure the mass of beaker + solution from step 5
7. Subtract the mass of beaker from the mass in 6 to get the mass of saturated solution
8. And evaporate as shown above to get the mass of crystals.
8. Measure the mass of crystal + beaker
9. Subtract mass of beaker from mass in 8 to get the mass of crystals
10. Subtract the mass of crystals from the mass of saturated solution to get the mass of water in the solution
11. I have assumed that all the masses are in grams. So, to get the solubility: Mass of crystal x 100/Mass of water

I've just started reading your notes, they have been really helpful. THANK YOU VERY MUCH.

I'm sorry if this question was asked before, but I just can't understand the formula u gave in point 11. The rest of the method seems fairly simple.

 

[smzimran]

All points assembled here:
Physics / Chemistry / Bio P5 help

 

[histephenson007]

And, can anyone tell me what are the usual GTs in paper 5 to get an A/A* for chem, bio and phy ?!

 

[smzimran]

And, can anyone tell me what are the usual GTs in paper 5 to get an A/A* for chem, bio and phy ?!

Around 18-20 I believe

 

[histephenson007]

Around 18-20 I believe

for an A?

 

[extremeranger]

hmmm... both make sense....but how come we divide Mr by 4..this makes it little impracticle..isnt it?

Dividing by 4 is incorrect.
Suppose, initially you have 1000cm3 and 50g. When you add these you get a 1 mol/dm3 solution because Mr= 50.
Now, when you divide both volume and Mr by 4, you're only decreasing the volume. The concentration remains the same.

Calculation shows this: Moles = 12.5/50 = .25 moles
conc = moles/vol = .25/.250 = 1 mol/dm3

We want a 0.5 mol/dm3 solution! So after dividing by 4, we must divide the mass by 2 because we want half the concentration. (Dividing by 4 only compensates for the decrease in volume from 1000cm3 to 250 cm3)

Its really confusing when you think about it like this. Just follow the conventional way of calculations like the one I showed above and you'll be fine

 

[smzimran]

for an A?

Yes!

 

[leadingguy]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_qp_5.pdf
http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_ms_5.pdf

qustion 2 part e)
can Anybody tel me that how to calculate the value of the charge???

 

[Abdulrab]

guys please solve this question 2 of p5 and kindly make graph for this question and other related questions to the graph should be solvedhttp://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_s09_qp_5.pdf

thanku waiting for a quick response

 

[angelgirl:)]

extremeranger
plz can u solve my doubt on the other thred i have posted...tips for bio,chem ,phy p5...

 

[candid24hours]

What is the best apparatus that can be used to find volume of gas, one which is workable and convenient. All I know is that there are two kinds of arrangements that can be employed: first, where we use thistle funnel and conical flask and second, where we use a small test tube containing for example an acid and we tilt it towards a strip of metal for the reaction.
Please kindly elaborate on this. I might not be totally right about this. But help.
Thanks. =)

 

[angelgirl:)]

how to calculate Q2d?

 

[angelgirl:)]

how to calculate Q2d?

y we have to divide by 2?

 

[leadingguy]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_qp_5.pdf
http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_ms_5.pdf

qustion 2 part e)
can Anybody tel me that how to calculate the value of the charge???

 

[histephenson007]

y we have to divide by 2?

I think it has something to do with the water being 50g as said in the question and the units all being /100g of water

 

[leadingguy]

http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_qp_5.pdf
http://www.xtremepapers.com/papers/...d AS Level/Chemistry (9701)/9701_w08_ms_5.pdf

qustion 2 part e)
can Anybody tel me that how to calculate the value of the charge???

 

[Sky8]

http://www.xtremepapers.com/papers/... AS Level/Chemistry (9701)/9701_w10_qp_51.pdf
For ques 2, isnt't it for point 3 which is above the line, the molality is less than expected , tat means too much water used and point 7 which is above the line , the molality is more than expected , reduced water used ? I'm confused with the marking scheme.

 

[saudha]

http://www.xtremepapers.com/papers/CIE/Cambridge International A and AS Level/Chemistry (9701)/9701_w10_qp_51.pdf
For ques 2, isnt't it for point 3 which is above the line, the molality is less than expected , tat means too much water used and point 7 which is above the line , the molality is more than expected , reduced water used ? I'm confused with the marking scheme.

u mean point 7 is below the line...