Hi all,

I want to add a powder to 100% propylene glycol  1 litre and make a 0.02% solution

How do i calculate that?

 

0.02% by weight is easy..   Weigh 0.02g of the powder for every 99.98g of the glycol. 

By volume you will need to include the densities. If you need the percentage to be molar then you will need the molecular weight of the chemical you are dissolving and Avogadro's number. Convert it all to moles and work out the ratios you need from there. - Same - 0.02 moles per 99.98 of the glycol.  

 

If you are formulating a lotion, where you don't need exact molar ratios because there are no chemical reactions taking place, then I would just work with masses  -  it is easier and quicker. For weighing out 1 L you just measure the density of the final product and work out how much you need to weight out to get 1 L.  

 

I hope this helps..  it depends on what you are exactly trying to achieve.