CHM 116 Quiz #5 1. You dissolve 5.00 g of sodium arsenate (Na3ASO4) in sufficien

Question

CHM 116 Quiz #5 1. You dissolve 5.00 g of sodium arsenate (Na3ASO4) in sufficient water to produce a final solution volume of 100.0 mL. Assuming all of the solid dissolves, calculate the solution pH. (10 pts) 2. You have available 1.00 L volumes of 1.00 M H,PO, 1.00 M NaH,PO, 1.00 M Na,HPO, and 1.00 M Na,PO. You wish to prepare 500.0 mL of a buffer solution that maintains a pH of 6.75. a) Indicate which two of these solutions you would use to produce this buffer, and calculate the volume of each of these two solution you would need to mix to get the desired volume and pH value for the buffer. (10 pts). b) Calculate the pH of the buffer solution, prepared in part a, after 10.00 mL 1.00 M NaOH is! added to it. (5 pts)

Explanation / Answer

The dissolution reaction of Na3AsO4 is:

Na3AsO4 + 3H2O ----------------> H3AsO4 + 3NaOH

Moles of Na3AsO4 = Mass of Na3AsO4 / Mol. Wt of Na3AsO4 = 5.00g/207.89 g/mol = 0.0241 moles of Na3AsO4

Accrding to the above reaction, for every 1 mole of Na3AsO4, 3 moles of NaOH are produced.

So for 0.0241 moles of Na3AsO4 , there will be 3*0.0241 moles of NaOH = 0.0722 moles of NaOH

So [OH-] = 0.0722 moles/0.100 L = 0.722

pOH = -log(0.722) = 0.142

pH = 14 - pOH = 14 - 0.142 = 13.86

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