buffer calculations - (Feb/06/2011 )

how can I prepare a 0.5 M solution of Phosphate buffer pH 7.2 (1000mls). I understand that I have to use the henderson-hasselbach equation for this. I was going to make a 1 M solution of the same and then dilute it.

But the information that I was given for this question is as follows.

mole weight of k2HPo4 =174.18 and mol weight of kH2Po4=136.06
to make phosphate buffer at required ph, use Xml of 0.2M KHpo4 and Y ml of KH2Po4 where Xml=36 and Yml=14,
I am not sure if I have to use this information

buffers can be prepared without using the henderson-hasselback equation.

your idea of making a higher concentration then diluting is common practice.

make up a volume of each phosphate (take care to include hydration in the formula weight) at 1M. then, take one and add the other until you reach the desired pH. don't worry about final volume (unless it is too little) because no matter the volume the concentration of phosphate will remain the same.

then dilute as needed.

ps- there are tables available that give the proportions necessary for various pHs and for various buffers.

15.6g monosodium phosphate monohydrate
104g disodium phosphate heptahydrate

Seems like you confusing to use stock, dilute or make new one??
dont worry here is the link how to prepare your desired solution concentration, by mass? by volume or by dilution

www.graphpad.com/quickcalcs/Molarityform.cfm

After diluting the 1 M solution, PH needs to be readjusted.