Making new solution from stock solutions

[rappa]

stock solutions: 100% acetic acid (liquid), 95% ethanol (liquid), dH20 (liquid)

1) 75 mL of Solution A: Fixing Solution (40% ethanol, 10% acetic acid)

they are saying if we have a 100% stock of acetic acid and 95% stock of ethanol, how do we make them into 40% ethanol and 10% acetic acid and combine to make 75mL

I was thinking maybe its: 31.6 mL of ethanol, 7.5 mL of acetic and 36 mL of water??

 

[Ygggdrasil]

Basically correct. Note that when mixing solutions, volumes do not always add up, so adding 31.6mL of ethanol, 7.5mL of acetic acid, and 36mL of water would not necessarily give you 75 mL. The best way would be to have some water in a beaker (~30mL maybe), add the ethanol, add the acetic acid, mix, then transfer to a graduated cylinder and add more water until the solution is 75mL.

 

[Borek]

Ethanol is a little bit off. See attachement - that's what CASC (concentration calculator) calculated.

Note: there are no 75 mL flasks (or at least they are very rare), so the idea of using measuring cylinder is better.

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[ChmDudeCB]

75ml * 0.10 (10%) = 7.5ml
75ml * 0.40 (40%) = 30ml
75ml * 0.50 (50%) = 37.5ml

When they give it to you in percentages it's easy. Just make sure everything adds up to 100% after you calculate each individual amount needed.

 

[Borek]

When they give it to you in percentages it's easy. Just make sure everything adds up to 100% after you calculate each individual amount needed.

If percentages are given as w/w it is not necesarilly that simple. Volumes are not additive.

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methods