highpower48 - 28-2-2018 at 08:29
Having a major brain fart. I need to dilute 70% ethyl alcohol to 50%. Common sense says if you have say 100ml of 70% all you need to do is add 30ml
water to make it 50%, but I can't make the math work out to prove it. Am I correct on the assumption of just adding the 30ml of water to make the 50%
solution?
Also can you use use isopropanol alcohol to make phenolphthalein?
Thanks
MrHomeScientist - 28-2-2018 at 08:34
M1V1 = M2V2
(70)(100) = (50)(x)
x = 140
Therefore, you need to add 40mL of water to your 100mL of 70% alcohol.
Also, you don't really need to censor 'fart'. 
I think most of us are adults
here. (?)
Diachrynic - 28-2-2018 at 08:36
Or you think of it like this:
100 ml 70% EtOH means 70 ml EtOH and 30 ml H2O.
Bring that equal to make 50% requires 40 ml H2O to be added to have 70 ml EtOH and 70 ml H2O
[Edited on 28-2-2018 by Diachrynic]
Sulaiman - 28-2-2018 at 08:38
100 ml 70% ABV contains 70 ml pure ethanol,
to dilute 70 ml ethanol to 50%ABV you need 70ml water, so 140ml total
You need to add 40ml water to 100ml 70%ABV ethanol to get 50%ABV
DJF90 - 28-2-2018 at 09:07
For best results, make the solution up to the desired volume (don't just add 40 mL - density changes can mean this leads to an inaccurate dilution).
aga - 28-2-2018 at 09:29
Rather than [M] in the formula, memorise "concentration1 x volume1 = concentration2 x volume2", i.e. :-
$$C_{1}V_1 = C_{2}V_2$$
where C is concentration ([M], w% whatever) and V is volume (ml, litres, pints etc).
In this case, you want the V2 term, so rearrange the formula to get:
$$V_2 = \frac {C_{1}V_1} {C_2}$$
stick in your values :
$$V_2 = \frac {70\text % \text x 100\text ml} {50\text %} = 140ml $$
So to get your 100ml of 70% EtOH to 50%, top up the volume with water to 140ml.
Exactly the same answer as MrHomeScientist's, just with some LaTeX 
[Edited on 28-2-2018 by aga]
MrHomeScientist - 28-2-2018 at 10:48
Whoops, looks like I misremembered the symbols for that equation! I think I went to M for molarity since that's what I usually work in.
But hey, pick any symbols you want as long as the math stays the same: [bear1][lemon1] = [bear2][lemon2]
Thanks aga.
aga - 28-2-2018 at 10:59
When you switch to Pints and w%, the maths goes all wrong at about 6 for the volume and definitely a max of 9 for the concentration, where Pints[beer]
x Conc[EtOH] = BrainState.
Here's the supporting data from a paper by aga et al, 1990,1991, 1992, thru 2018
[Edited on 28-2-2018 by aga]