If you have a crude plant extract containing a mixture of substances, the concept of molar concentration is meaningless unless you are interested in one single compound in that mixture. In that case, you need to know both the concentration in terms of mg/mL and the molecular weight.

You can prepare molar solutions from stock solutions whose molecular weight is known.

You can not prepare mM solutions from the plant extracts. But you could prepare the mg/ml based on the weight of the concentration of the plant extracts.

But you could prepare the mg/ml based on the weight of the concentration of the plant extracts.

Know the difference between molarity, normality and percentage solutions which is the basis

for biochemistry

Thank you PROF. Kantha, I do prepare my concentration for in vivo as mg/ml but when I was asked to prepare xmM for invitro assay knowing fully well the molecular weight (g/mole) is not known. There comes the issue. Thanks 

Dear Sir,

Maybe for in-vitro assays if you are using the standard bioactive component in the plant extract then you could prepare the mM solution from the molecular weight given I the bottle.

Instead, if you are using the plant extract you can not add mM solution instead you have to calculate how much of the active substance present in the plant extract and calculate accordingly as mg/g of the plant extract and add. (The amount of active component you could back calculate from your HPLC analysis.)

I'd like to float the idea of expressing plant extracts as nominal composite molar content. From GCMS one has a list of compounds with molecular wieghts, area under the curve of each expressed as a percentage of the total, and the total weight of extract measured. This would give an idea of the molar content of the extract per gram of material. This would be more meaningful for expressing potency in in vitro assays. Comments anyone?