Help Me With These Calculations Please!!!!

[Soniakhan]

I have to do alot of calculations and most of them are on same pattern but I dont know how to solve them.. Please if anyone help me understand that. for example,

1). What weight of potassium permanganate is required to prepare 250ml of a solution such that 10 mL diluted to 1 litre will give a 1 in 10,000 solution?
A 2.5g
B 10 g
C 25 g
D 100 g
E 250 g

2). To prepare 5 litres of a solution containing potassium ions 1 mmol/mL. What weight of potassium chloride is required?

(atomic weight of potassium = 39: atomic weight of chlorine = 35.5)
A 0.3725 g
B 0.745 g
C 37.25 g
D 74.5 g
E 372.5 g

[Trishy]

2) 1mol KCl = 39 + 35.5g = 74.5g 1mmol KCl = 74.5/1000 = 0.0745g per ml
5 litres = 5000ml
Wt of KCl required for 5l = 0.0745 x 5000 = 372.5g
Answer E

[Trishy]

1) We have to work backwards to calculate the weight.
a 1 in 10,000 solution contains 1g in 10 litres which is the same as 0.1g in 1 litre.
10 ml containing 0.1g is diluted to 1 litre to make the required final solution.

250ml of this solution contains 250/10 x 0.1g = 2.5g
Answer A
I hope that's right and makes sense - my brain hurts now!!

[Soniakhan]

Thank you so much :)