Help w chem question (1 Viewer)

inkjoy11

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6) What mass of the indicated substance do you need to weigh out to make the following solutions?
a ) Barium hydroxide to make 0.5 L of 0.060 mol L–1 hydroxide ion solution
 

jazz519

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Find moles using c=n/v

The question says for a 0.060 mol/L OH- solution, so we calculate moles for OH-

n(OH-)= cv = (0.50)(0.060) = 0.03 moles

In terms of relating to the moles of barium hydroxide. Two atoms of OH- are used to make 1 Ba(OH)2

So therefore we divide the number of moles of OH- by 2, giving:

n(Ba(OH)2) = 1/2 x n(OH-)
n(Ba(OH)2) = 0.015 moles

find mass using n=m/MM

m(Ba(oh)2) = n x MM = (0.015)(137.3+2(16+1.008))

Sub number from here
 
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Velocifire

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Thanks for the explanation Jazz, like showing which formulas to use.

So am I right if
n = no of moles
c = concentration Moles/l
v = volume (litres)
m = mass in terms of grams
mm = molar mass

ps, damn, I rounded the atomic masses of the numbers when I was attempting the question like the hydrogen was rounded to 1, oops.
 

inkjoy11

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Find moles using c=n/v

n=cv = (0.50)(0.060) = 0.03 moles

find mass using n=m/MM

m(Ba(oh)2) = n x MM = (0.03)(137.3+2(16+1.008))

you can sub in the numbers to evaluate that yourself

Thanks Jazz,
I was really unsure about it because the answer my teacher posted was the following.

n = cV
n (Ba(OH)2) = 0.060 x 0.5 = 0.03 mol
n(OH)2 = 0.03/2 x 0.015
m = 0.015 x (137 + 2 x (16 + 1.0)) = 0.03 x 171 = 2.6 g

I have no idea what she is doing and the last line does not even work out to be correct. Any idea why she divided the number of moles by two?
 

jazz519

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Thanks for the explanation Jazz, like showing which formulas to use.

So am I right if
n = no of moles
c = concentration Moles/l
v = volume (litres)
m = mass in terms of grams
mm = molar mass

ps, damn, I rounded the atomic masses of the numbers when I was attempting the question like the hydrogen was rounded to 1, oops.
Yep that’s correct
 

Velocifire

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Probably the teacher got confused, but there is a reason since it does say moles per litre and they wanted 500ml? That's probably why it was divided by two? Not too sure, don't quote me on this.

Edit: I was stumped, they were correct.
 

jazz519

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My bad I've fixed the above answer kind of skim read it the first time. That should explain to you why we divide by 2
 

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