Help! Chad's Video Quiz Problem involving pH and Ka

[Strag]

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What is the pH of 0.1M HCN (Ka=5.0 x 10^-10)?

a) 5.2
b) 4.9
c) 6.5
d) 3.8

Answer: A

[H+]= sq rt Ka x [Ha]= sq rt (5 x 10^-10)(0.1)=sq rt (5 x 10^-11)=sq rt (50 x 10^-12)=7 x 10^-4

pH=-log [H+]=-log (7 x 10^-4)

Ok so I arrived at the last thing. What I'm confused is shouldn't the answer be something less than 4? The [H+] is higher than 1 x 10^-4 which would correspond to a pH value of 4, therefore, the pH value should be less than 4.

So shouldn't the answer be D?

 

[recyrb]

What is the pH of 0.1M HCN (Ka=5.0 x 10^-10)?

a) 5.2
b) 4.9
c) 6.5
d) 3.8

Answer: A

[H+]= sq rt Ka x [Ha]= sq rt (5 x 10^-10)(0.1)=sq rt (5 x 10^-11)=sq rt (50 x 10^-12)=7 x 10^-4

pH=-log [H+]=-log (7 x 10^-4)

Ok so I arrived at the last thing. What I'm confused is shouldn't the answer be something less than 4? The [H+] is higher than 1 x 10^-4 which would correspond to a pH value of 4, therefore, the pH value should be less than 4.

So shouldn't the answer be D?

sqrt -12 = -6

 

[Strag]

sqrt -12 = -6

Awesome caught my mistake thanks for the help!😀