# Converting mg/mL to sample mass?

#### BillyWu

Hi everyone. I've got a problem that I've never been taught the solution to and I'm having difficulty in finding a clear answer. Here is my problem.

I have an aspirin tablet and I want to find out if the amount of aspirin content in the tablet is the same as advertised on the box. It was accurately weighed to 0.3422g (342.2mg). I used reverse phase HPLC as my method of analysis.

• 250mg pure aspirin was dissolved into an eluent in a 100mL flask and then diluted to produce 0.10, 0.15 and 0.20mg/ml concentrations. These were used to obtain reference peak heights for a calibration curve.
• 0.025g of the aspirin tablet was dissolved in 10ml of an eluent (the eluent being a 1% glacial acetic acid and HPLC grade methanol solution in a 6:4 ratio)
• Using a calibration curve in MS Excel with the known peaks from the pure dilutions and the peak height from the unknown aspirin tablet, I obtained the concentration of the unknown.

The concentration of the unknown tablet was 2.62mg/ml aspirin. The total tablet mass was 0.3422g (342.2mg).

How do I work out the amount of aspirin in the tablet with this information?

#### BillyWu

This question might be better suited in the advanced chemistry section, seeing as it's analytical chemistry. It's for an undergraduate degree though so I thought this was the right place for it

#### Unknown008

CHF Hall of Fame
Well, it is a bit strange of a result...

2.62 mg/mL means 0.00262 g/mL

Since you used 10 mL of eluent (solute), this means total mass is 0.00262*10 = 0.0262 mg in 10 mL

Which is more than what you dissolved in the first place!

#### BillyWu

Exactly! My lecturer told me to expect abnormal results but this is pretty confusing.

I have tried using this formula, if anyone can verify that it is correct?

(tablet mass / sample mass) x (sample conc x 10)

#### bjhopper

Hi everyone. I've got a problem that I've never been taught the solution to and I'm having difficulty in finding a clear answer. Here is my problem.

I have an aspirin tablet and I want to find out if the amount of aspirin content in the tablet is the same as advertised on the box. It was accurately weighed to 0.3422g (342.2mg). I used reverse phase HPLC as my method of analysis.

• 250mg pure aspirin was dissolved into an eluent in a 100mL flask and then diluted to produce 0.10, 0.15 and 0.20mg/ml concentrations. These were used to obtain reference peak heights for a calibration curve.
• 0.025g of the aspirin tablet was dissolved in 10ml of an eluent (the eluent being a 1% glacial acetic acid and HPLC grade methanol solution in a 6:4 ratio)
• Using a calibration curve in MS Excel with the known peaks from the pure dilutions and the peak height from the unknown aspirin tablet, I obtained the concentration of the unknown.

The concentration of the unknown tablet was 2.62mg/ml aspirin. The total tablet mass was 0.3422g (342.2mg).

How do I work out the amount of aspirin in the tablet with this information?
An aspirin tablet weighs about 2 gms. Assay of aspirin content would involve the whole tablet.It is not clear how you prepared the aliquot for determing asp content,there are 325 mg asp in tablet

#### Unknown008

CHF Hall of Fame
Exactly! My lecturer told me to expect abnormal results but this is pretty confusing.

I have tried using this formula, if anyone can verify that it is correct?

(tablet mass / sample mass) x (sample conc x 10)
Well, if that is acceptable I guess...

And for your formula, I would say yes. Just don't forget to consider the units. I would advise however not to put everything on one line and do the calculations as clearly as possible. This will avoid any confusion if ever you have to look back for corrections or to change anything.