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# %w/v watch

1. Could somebody explain the following example and how to work it out? I'm really confused.

"What is the %w/v of 0.5M aspirin (MW = 180.2)?"

I know %w/v is g/100ml but I'm struggling to understand how to turn the above question into a value.

Thanks.
2. You have a concentration - 0.5 mol dm-3 = moles/volume

you need to convert it to g/100ml = mass/volume

to connect the two use moles = mass/Mr

then it's just playing around with the volumes

3. Ah, right, I see. Could you double check this for me, EVS?

Moles = mass/mr rearranged into moles x MR = mass.
-> 0.5 x 180.2 = 90.1g of aspirin in 1000ml.

90.1/10 = 9.01 in 100ml which is 9.01%w/v?
4. Looks good
5. Yessssssssssss. Thanks for your help, EVS.
6. Another question I can't seem to get my head around:

A sample of 0.1034 g of sodium carbonate (relative molecular mass 105.99) is titrated against 20.41 mL of hydrochloric acid solution. What is the molarity of the acid?

I keep getting half of the answer using -

moles = mass/Mr = 0.1034/105.99 = 9.76x10-4

Then using concentration = Moles / V x 1000 = 0.1034/20/41 = 4.78 x10-5 x 1000 = 0.0478 Moles.

The answer should be 0.0956M. I feel like I know it but can't put my finger on it and it's really irritating me.
7. Consider the equation:

Na2CO3 + 2HCl ---> 2NaCl + CO2 + H2O

8. Ah, I think I get it. Would the correct way be doubling the moles of Na2CO3 to make the ratio 1:1?
9. You just know from the equation that for every mole of carbonate there has to be twice as many moles of HCl, so just times your answer by 2 at the end to take into account the 2:1 ratio as per the equation
10. That makes sense now, thanks again EVS. I'll let you know if I've got any more questions

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