June 07, 2020, 03:08:39 AM
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### Topic: Question from my supervisor RE making a soln.  (Read 214 times)

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#### pardyhoo

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##### Question from my supervisor RE making a soln.
« on: April 03, 2020, 11:33:02 AM »
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?

#### chenbeier

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##### Re: Question from my supervisor RE making a soln.
« Reply #1 on: April 03, 2020, 11:59:02 AM »
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

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##### Re: Question from my supervisor RE making a soln.
« Reply #2 on: April 03, 2020, 05:32:48 PM »
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.