[pardyhoo]

Hey Chemical Forum, I got a take home question from my supervisor at work and would very much love learn how to solve it.

How to make 4L of 100mM NaH2PO4.6H2O + 8mM TBAHS w/ 3% Methanol . pH3.5

I believe the Molar mass of NaH2PO4.6H2O is 251.0542 g/mol and TBAHS is 339.53g/mol.
So 25.1g of NaH2PO4.6H2O would be 100mM and 2.72g of TBAHS would be 8mM.
And 3% of 4L is 120mL
Should I add 25.1g of NaH2PO4.6H2O, 2.72g of TBAHS, 120mL of Methanol and top the content of to 4L with H2O?

Cheers in advance !

[chenbeier]

First you have to dissolve the TBAHS in Methanol,  otherwise you would have precipitation. The Phosphate also has to dissolved in water first. Then you combine both solution.

[mjc123]

25.1g is 100 millimoles, not 100 mM.

The symbol M means moles per litre (concentration), not moles (amount).

For 4 L of 100 mM solution, you need 400 mmol. Similarly with the TBAHS.