Standard prep using liquids

[mrbackflush]

Hello; new member here. I have been tasked with making standards from liquid stocks obtained from a supplier.
I am also not the best at stoichiometry.
I need to prepare a 10,000 ppm solution of 3-Pentanol. The final volume needs to be 500 mL. Here is what I came up with:
1. Calculate the mass of the solute needed: Mass of solute (mg) = (Desired ppm concentration) x (Volume of solution in mL) / 1,000,000.
(10,000 mg/L)(500 mL)/1,000,000 = 5.0 mg
Volume needed = mass of solute/(purity as decimal) (density in g/mL)
The 3-Pentanol I have is 98% pure, density = 0.815g/mL.
5.0mg/0.7987 = 6.26mL needed in 500mL.
I was taught that density must be included in the calculations when making solutions from liquid solutions. The method I was given uses weight only for making this solution from a liquid.
Thank you in advance!

[Hollow]

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)

[Peter Apps]

"ppm" could mean mg/l or mg/kg or microloles/mole and depending what your solvent is the quantity of solute will be different in each case.

The first step is to work with proper units, then the calculation will be more straightforward.

Peter

[mrbackflush]

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?

[mrbackflush]

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?

[Hollow]

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?

yes in the end it was ok, because you've made two times an error by 1000x, which luckily canceled out.

but next time it may not, so be more careful with the units and write them down properly.

BTW: we should close this thread and only keep the other one under. https://www.chromforum.org/viewtopic.php?t=122226

In general, don't open the same question multiple times in different sub-forums, as it creates a mess

[mrbackflush]

[/quote]
In general, don't open the same question multiple times in different sub-forums, as it creates a mess
[/quote]

OK - we can continue on the other sub-forum - thanks for letting me know.

[mrbackflush]

[/quote]
In general, don't open the same question multiple times in different sub-forums, as it creates a mess
[/quote]

OK - we can continue on the other sub-forum - thanks for letting me know.