Help w chem question (1 Viewer)

[inkjoy11]

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]

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

 

[Velocifire]

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]

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]

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]

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]

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