I have a genuine question about Chemistry pertaining to Stoic.

[wildareal]

Hi there,

What mass of Barium Chloride (BaCl2) will remain after a 15.0g sample of the hydrated salt BaCl2.2H20 is heated to drive off all of the water.

Thanks.

[luken93]

n(BaCl2.2H2O) = 15/???
m(H2O) = 15/??? x 2 x 18
m(BaCl2) = n(BaCl2.2H2O) - m(H2O)

soz i dont have the figures, but this should make it pretty clear

[stonecold]

Mr(BaCl2.2H2O)=244.3 g/mol

n(BaCl2.2H2O)=15.0/244.3 = 0.0614 mol

n(H2O)= 2 x n(BaCl2.2H2O) = 2 x 0.0614 = 0.123 mol

m(H2O)= 0.123 x 18 = 2.21 g

m(BaCl2)= original mass - water mass = 15.0 - 2.21 = 12.8 g

(only 1 d.p. is required)

[m@tty]

Or you can just figure out the proportion of the mass that barium chloride is, then multiply that by the mass.







So

And

Therefore

[luken93]

Can I just confirm something about sig figs, it's 3 because 15.0 is three numbers starting left to right from 1?
And 1 d.p because it also has .0?

[Greatness]

Can I just confirm something about sig figs, it's 3 because 15.0 is three numbers starting left to right from 1?
And 1 d.p because it also has .0?

I think that's correct.

[m@tty]

Can I just confirm something about sig figs, it's 3 because 15.0 is three numbers starting left to right from 1?
And 1 d.p because it also has .0?

With addition and subtraction go for the lowest number of decimal places.

With multiplication and division go for the least number of significant figures - that is the number of digits between the first and last non-zero number (inclusive).

[luken93]

So do you mean when you are doing working out do this for every step?

[Greatness]

I believe you can more than the required signifcant figures for you're working out, but you're final answer must be  in the no. of signifcant figures or decimal places stated in the question.

[wildareal]

Why is it in hydrated substances you times the figure by the mole ratio (eg 2 in 2H20) when calculating the Molecular weight when in normal stoic questions you do not?

[m@tty]

The hydration factor is the number of molecules of water per molecule of the substance. So to figure out the mass of water you need to multiply M(H2O) by 2 because there are two molecules which must be counted in the molecular mass.

And luken, it's only relevant to the final answer working can be to as many decimal places as you like - I'd generally keep it at 4 throughout working. Plus, there is only one question per exam where significant figures is required I think. It specifies it in the question "find XXXX to an appropriate number of significant figures."

[luken93]

And luken, it's only relevant to the final answer working can be to as many decimal places as you like - I'd generally keep it at 4 throughout working. Plus, there is only one question per exam where significant figures is required I think. It specifies it in the question "find XXXX to an appropriate number of significant figures."

OK fair enough, but then when do you apply these rules?

do you mean to say that if you need to find n = cV in the final part of the question you go by the least number of sf

ie. n = 0.1M x 1.23L it would be .1 mol/L?

If that's right what is an example of adding/subtracting then?

[Greatness]

I think this may help:
Addition and subtraction: The sum of 2 no.s is written to the smallest decimal place. I.e. 2.34 + 114.5 would equal 116.5 (This is 4 significant figures)
Multiplication and division: The product of the no.s must be written as the lowest no. of significant figure in the data. i.e. 1.7, 34.567, 0.887
1.7*34.567*0.887=52.124
But the answer has to be correct to 2 significant figures. Thus 5.2*10 is the correct answer.