Edexcel CHEMISTRY DOUBTS!!!!

[Summer :]]

How do we calculate the solubility of a substance???

grams dissolved in the water
its just the basic equation of moles that you should use
no of moles = mass/ Mr
find the mass..thats all..

[Summer :]]

i have a question about percentage error.....

The balance used to weigh the magnesium carbonate is accurate to +-0.01 g. Calculate the percentage error in the mass of the magnesium carbonate weighed. ((weighed MgCO₃: 0.21g))

the correct answer is 0.02
                             -----
                               0.21

Why is it 0.02 not 0.01?? and when do we write it as 0.01??   

Thanks in advance peeps

because
it says +- 0.01
therefore the range could be 0.21 - 0.01
and 0.21 + 0.01
so add those values together 0.01 + 0.01 = 0.02
its always this way i guess..thats what my teacher told us *shrugs* 

[ksitna]

So in Acid-Base equilibrium when were trying to find the pH of a buffer solution, is the rule:

pH = pKa + log([A ^– ] ÷ [HA])

OR

pH = pKa - log([A ^– ] ÷ [HA])

Because in the book they're using the one with the -log and in some questions in the pastpapers they're using the +log. Which one is it exactly?

we have to use log in AS chemistry???

[luluabdl]

we have to use log in AS chemistry???

Sorry wrong post, this is A2

[ksitna]

Sorry wrong post, this is A2

ohh okie
jeez man u freaked me out!!!!

[ksitna]

i need help with percentage yields
can someone help me?

[Tammy]

because
it says +- 0.01
therefore the range could be 0.21 - 0.01
and 0.21 + 0.01
so add those values together 0.01 + 0.01 = 0.02
its always this way i guess..thats what my teacher told us *shrugs* 

Alright!!! Thanks!!!

[Summer :]]

Alright!!! Thanks!!!

your welcome! By the way i just asked about
if it says percentage error
then you
(2 x error) / divide by whatever mass X 100
like i said previously
if it says its an error (not percentage)
multiply error by the mass or volume provided / 100

[Summer :]]

does anyone know how to solve the last question on Jan '09 paper (old syllabus)?
I'll post the question if anyone is bothered or is not busy

[Saladin]

I will answer.

Please state the paper number.

[coatseville]

i need help with percentage yields
can someone help me?

For this you use the formula actual yield / theoretical yield x 100.

[ksitna]

thanks

and can someone tell me where i can find specimen unit 3b?

[Blizz_rb93]

Can someone help me with this question from Jan 2010 unit 3b..
Question 4. branch f) only

Here's the question paper and markscheme attached below

[Meticulous]

Please any body...a quick question.

When adding alkali to acid, it says:

"Swirl / rinse in drops solution in neck of flask (1)"

what is that supposed to mean?

[Saladin]

Please any body...a quick question.

When adding alkali to acid, it says:

"Swirl / rinse in drops solution in neck of flask (1)"

what is that supposed to mean?

This basically means that when adding an acid to a base, you need to keep swirling it. Move it and jerk it in a circular motion. Because often, drops dont go straight to the liquid, the often stay on the side of the conical flask.