Thanks a lot:)
So just to confirm: normally for any chemical bought from manufacturer, if it is not stated, the % usually means v/v? Seems like for conc sulphuric acid, when it is stated 97% sulphuric acid, it means 97% w/w sulphuric acid?
Thank you very much.
Posted by annie on Monday, September 20, 2010 at 10:33am.
I have a bottle of 95% ethanol purchased from the manufacturer.
Normally, when it is not stated, does it mean 95% w/w ethanol?
I need to prepare 100ml of 40% v/v ethanol solution from 95% ethanol.
Please show me the calculation steps.
Thanks.
Chemistry -repost thank you in advance - bobpursley, Monday, September 20, 2010 at 11:01am
when not stated, 95% is v/v
you want to dilute the ethanol 95/45 times, or 2.111 times. THat means one part alcohol, and 1.111 parts water.
100ml/2.111=47.37 is a part.
so, 47.37 ml of 95%
52.63 ml of water.
I think your conclusion is wrong. You are correct that the percent on a bottle of H2SO4 means w/w. Technically, ALL manufacturers (and problem makers) should state 95% v/v or 98% w/w (or 95% w/v) but they don't. USUALLY one means w/w if it isn't stated but USUALLY one means v/v if it is two liquids (as in 95% ethanol)(which flies in the face of the H2SO4 bottle).
To confirm, when the percentage of a chemical is not specified, it usually means v/v (volume/volume). However, for concentrated sulfuric acid, when it is stated as 97%, it refers to w/w (weight/weight) percent.
For the case of the 95% ethanol you have purchased, since it is not stated, it can be assumed to be 95% v/v ethanol.
To prepare a 100ml solution of 40% v/v ethanol from the 95% ethanol, you can follow these calculation steps:
1. Determine the dilution factor: The dilution factor is the ratio of the final volume (100ml) to the initial volume of 95% ethanol. In this case, the dilution factor would be 100ml divided by the volume of 95% ethanol.
2. Calculate the volume of 95% ethanol needed: To calculate the volume of 95% ethanol needed, divide the final volume by the dilution factor. In this case, 100ml divided by the dilution factor of 2.111 (95/45) gives you 47.37ml.
3. Calculate the volume of water needed: To calculate the volume of water needed, subtract the volume of 95% ethanol from the final volume. In this case, 100ml minus 47.37ml gives you 52.63ml.
So, to prepare 100ml of a 40% v/v ethanol solution from 95% ethanol, you would need to mix 47.37ml of 95% ethanol with 52.63ml of water.