solution help with the maths

[Narelle01]

I just made phenolpthalein according to the instructions in the manual - school assistants working in science.
5gms in 500ml of ethanol, add 500mls water. Stir

But i halved it.....so 2.5gms in 250mls, then added 250mls water.

But is this solution not 1% now??? is is more like 0.5% solution - or am i having a dumb day and should go home and not touch any more chemicals????

[kimmy]

my receipe is
2g phenolphthalein in 600ml ethanol then make up to 1 litre with distilled water and filter.

but if you have halved your instructions than it should still me the same M.

But once again it is Monday morning so anything is possible.

KIMMY

[Narelle01]

thanks - the agar seemed to work okay when i added the NaOh...it is a deep colour - i just keep telling myself it is a senior prac - not an exam - or a uni thing....
Thanks for your encouragement

[mtg]

If it's percentage then 1g into 100ml is 1% so I reckon you're right. But it'll probably be ok as an indicator. I use 0.1% and 1% solutions and it"s only the water, wine, milk , beer/champagne prac (that I do) that it matters on the strength. So if you really need 1% start again with 125 and 125mls, trying to do the sums to rectify your existing soln is too hard on a monday.

[RosalieM]

If your original was 1% and you halved the whole lot then it will still be 1% but your original is 0.5% (because it is 5g in 1000ml - 1% would be 10g in 1000ml) so your smaller lot is also 0.5%

[lada]

Narelle, your original solution is only 0.5%.(5g into 1L.)
Indicator is not that critical anyway, it is only indication of colour. Anywhere from 0.1-0.5% will work.
Lada