Chromatography Forum

Hi mrbackflush

almost... in principal the calculation process is ok but watch out the units!

and even liquids can be weight in, which is more accurate than by volume. be aware the density has its uncertainty, then your pipette will have, and you would need temperature control to 20°C... so weighing is much better in this case.

Second:
in the first calculation, there is an error of a factor 1,000!
as you already taken ppm as "mg/L", no need to divide by 10^6.

If you have problems with such calculations, always write them as classical fractions on a big fraction bar and resolve any double-division. Then second, the SI-prefixes are just abbreviations for 10^x


so if you write 500 ml * 10,000 mg/L like

Code: Select all

            500*10e-3 L * 10,000*10e-3 g
mass = ------------------------------
                           1 L

you can cancel the "L" and are left "g" and the number of 5,000,000 * 10e-6 = 5, so 5 g. (not 5 mg)

(also makes sense if you cross check: 10,000 mg/L; and you're gonna prepare half a liter, so 5,000 mg)

So 5 g of pure substance, so divide by 0.98 g/g to know how much of the "unpure" substance.

In your calculation with the density, you've made a similar mistake by the factor 1,000

Again write it on a big fraction mark and

Code: Select all

         mass           5 mg
Vol = --------- = --------------;
        density         0.815 g / ml

-> double fraction, so "ml" ends up above the fraction bar

Code: Select all

     5 mg * ml      5*10e-3 g * 1*10e-3 L
 = --------- = ------------------------
        0.815 g         0.815 g

now "g" cancels and you're left with "L" and 5*10e-6, so 5 µl (not "ml" like you wrote; but your 5 mg was wrong and should have been 5 g)

Hi mrbackflush

almost... in principal the calculation process is ok but watch out the units!

and even liquids can be weight in, which is more accurate than by volume. be aware the density has its uncertainty, then your pipette will have, and you would need temperature control to 20°C... so weighing is much better in this case.

Second:
in the first calculation, there is an error of a factor 1,000!
as you already taken ppm as "mg/L", no need to divide by 10^6.

If you have problems with such calculations, always write them as classical fractions on a big fraction bar and resolve any double-division. Then second, the SI-prefixes are just abbreviations for 10^x


so if you write 500 ml * 10,000 mg/L like

Code: Select all

            500*10e-3 L * 10,000*10e-3 g
mass = ------------------------------
                           1 L

you can cancel the "L" and are left "g" and the number of 5,000,000 * 10e-6 = 5, so 5 g. (not 5 mg)

(also makes sense if you cross check: 10,000 mg/L; and you're gonna prepare half a liter, so 5,000 mg)

So 5 g of pure substance, so divide by 0.98 g/g to know how much of the "unpure" substance.

In your calculation with the density, you've made a similar mistake by the factor 1,000

Again write it on a big fraction mark and

Code: Select all

         mass           5 mg
Vol = --------- = --------------;
        density         0.815 g / ml

-> double fraction, so "ml" ends up above the fraction bar

Code: Select all

     5 mg * ml      5*10e-3 g * 1*10e-3 L
 = --------- = ------------------------
        0.815 g         0.815 g

now "g" cancels and you're left with "L" and 5*10e-6, so 5 µl (not "ml" like you wrote; but your 5 mg was wrong and should have been 5 g)

Thanks; I had another tech say my calculation was correct, but you say I'm off by a factor of 1000. What volume, in mL, am I supposed to add to 500mL of solvent?