13) A student is trying to prepare 350mL of 0.25M NaClI from a stock solution of

Question

13) A student is trying to prepare 350mL of 0.25M NaClI from a stock solution of 2.50M NaCI. He takes 35mL of 2.50M NaCl and adds 350mL of water to it. What did he do wrong? 14) A student dissolved 0.1625g of oxalic acid (C2H2O4·2H2O) in 25.00mL of DI water and titrated 25.78mL of 0.1000M KOH. How much KOH would be needed if the oxalic acid was dissolved in 15.00mL of DI water? 15) For each of the following compounds, draw the Lewis dot structure, and then determine the number of regions of electron density around the central atom, the electronic geometry, and molecular geometry: a) BCI3 b) CF4 c) 13- d) CO2 e) OF2 f PH3 9) XeF4 h) XeF202 PF5

Explanation / Answer

13) The student took 35.0 mL of 2.50 M NaCl and added 350.0 mL water. The final volume of the solution is (35.0 + 350.0) mL = 385.0 mL.

Use the dilution equation to find out the final concentration of NaCl.

M1*V1 = M2*V2 where M1 = 2.50 M; V1 = 35.0 mL and V2 = 385.0 mL; therefore,

(2.50 M)*(35.0 mL) = M2*(385.0 mL)

====> M2 = 0.227 M.

The student wished to prepare a 0.250 M NaCl solution; but instead prepared a solution that was 0.227 M in NaCl, i.e, he prepared a more dilute solution. He should have added (350.0 -35.0) mL = 315.0 mL of water to 35.0 mL of 2.50 M NaCl to prepare the 0.250 M solution (ans).

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