[mp420]

Could someone please check this question that i'm working on.

Calculate the molarity (M in mol dm^-3) of:
a) 2 moles H₂SO₄ dissolved in 250cm³ of water
b) 20g NaOH dissolved in 500cm³ of water

i used the formula: concentration = no. of moles /volume (in dm³
so for part a) i did: (98.1 x 2) / 250
= 0.7848

For part b):
no. of moles of NaOH = 20/40
= 2 moles
then 2/500 = 4x10^-3
Is that the molarity or do i need to sleep before i attempt anymore chemistry?

[DartsOfPleasure]

I'm a little confused on why you have 98.1 x 2 in part a)

Wouldn't the working out be: molarity = moles/volume

molarity = 2moles/0.250dm3  (you have 250cm3 and so you have to divide by 1000 to get that into dm3)

= 8M

For part b) You need to do moles = mass/molar mass

moles = 20grams/40 = 0.05

Therefore there are 0.05 moles in sodium hydroxide. You now use the formula molarity = moles/volume

molarity = 0.05/0.500 which equals 0.1M

Hope this helps!  

[mp420]

Thanks it's all clear to me now!

[Borek]

20grams/40 = 0.05