gen chem question

[busdent]

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How many milliliters of 0.05 M HCl are required to turn 20 ml of 0.05 M KOH into a solution of pH 2.00?



A. 10
B. 15
C. 20
D. 25
E. 30


The answer is E. I don't understand how..Thanks

 

[Djapprentice]

How many milliliters of 0.05 M HCl are required to turn 20 ml of 0.05 M KOH into a solution of pH 2.00?



A. 10
B. 15
C. 20
D. 25
E. 30


The answer is E. I don't understand how..Thanks

This problem is from Achiever 2, ok, there's a problem similar to this in Topscore 2, # 68 & another one at the end of the acids & bases chap, the review questions section. The way a girl & I came up with this at kaplan was that first, you're neutralizing the acid & the base, so you would use the titration formula: N1V1 = N2V2, then you're diluting the acid-base solution with more acid, hence the pH = 2. I'm still working on visualizing this problem so it becomes second nature to me. As you can see from the solution, the titration formula has to be used twice, & in the second titration formula, you're adding more volume, so the variable you're solving for is taking that into account. This is kind of hard to explain, I hope that helps, if not let me know, & we'll try it from another angle. Happy studying!

 

[busdent]

This problem is from Achiever 2, ok, there's a problem similar to this in Topscore 2, # 68 & another one at the end of the acids & bases chap, the review questions section. The way a girl & I came up with this at kaplan was that first, you're neutralizing the acid & the base, so you would use the titration formula: N1V1 = N2V2, then you're diluting the acid-base solution with more acid, hence the pH = 2. I'm still working on visualizing this problem so it becomes second nature to me. As you can see from the solution, the titration formula has to be used twice, & in the second titration formula, you're adding more volume, so the variable you're solving for is taking that into account. This is kind of hard to explain, I hope that helps, if not let me know, & we'll try it from another angle. Happy studying!

thanks DJ


want to try this one ? thanks

What is the molality of 85% phosphoric acid, H3PO4, if the density of the solution is 1.70 g/ml?



A. 1000 * (1.70/1) * (100.0 / 15.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
B. 1000 * (1/1.70) * (100.0 / 15.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
C. 1000 * (1/1.70) * (100.0 / 15.0) * (1/1.70) * (85.0 / 100.0) * (1/98) m
D. 1000 * (1/1.70) * (15.0 / 100.0) * (1.70/1) * (85.0 / 100.0) * (1/98) m
E. 1000 * (1/1.70) * (100.0 / 15.0) * (1.70/1) * (100.0 / 85.0) * (1/98) m


Achiever doesn't explain clearly why the answer is B