I have a physical chemistry question?

Question

'your chemistry tutor has asked you to make up 60ml of 0.08 M Mg2+ in solution. Still a bit unsure of this aspect of chem, you add 10 ml of 0.009 M MgCl2 and 50ml of Mg2(PO4)2. what is the concentration of Mg2(PO4)2 required to achieve the 0.08 M solution?

Answer 1

to make Mg2+ this way is not correct ,

you can try making CH3COOMg solution

CH3COO2Mg  >>>>>> 2CH3COO-  + Mg2+

to have 60ml of 0.08 M Mg2+  (4.8*10-3mol) solution you'll need  4.8*10-3mol of CH3COO2Mg solution & to make that you need to react CH3COOH with Mg(OH)2

Answer 2

Actually,  Mg2(PO4)2  does not exist .  The available chemical is  " Mg2P2O7 "  ( Soluble in Acidic Solution )  

Mg++  Contribution  From  0.009 M MgCl2  ( 10 mL )  to  Final Solution ( 60 mL )  =  0.009 M x 10 / 60  >>  0.0015 M  

Mg++  Concentration Contribution  From  " ? " M Mg2(PO4)2  ( 50 mL )  to  Final Solution ( 60 mL )  =  " ? " M x 2 x 50 / 60  >>  " ? " x 10 / 6 M  

0.0015 + ( 10 / 6 ) ?  =  0.08    --------- >  ?  =  ( 0.08 - 0.0015 ) / ( 10 / 6 )    -------- >  ?  =  0.0471 M