How Do I find the Concentration of H+ when I'm given pH?

[PreMade]

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How can I do this without a calculator?

I found some problems online, but have trouble understanding.

If a soil has a pH of 4.7, what is the H+ concentration of the soil solution?

• 
  pH = -log [H+]
  4.7 = -log [H+]
  4.7 = -(- 5) - 0.3; 100.3=2.0 (the -5 tells you the number of positions to move the decimal point to the left of 100.3=2.0)
  
  [H+] = 0.00002 M/l
  
  or place -4.7 in your calculator and take the antilog (often inverse log or 10x) = 0.00002 M/l
  
  
  
  
  
  Example: What is the hydronium ion concentration in a solution that has a pH of 8.34?
  
  
  
  8.34 = - log [H3O+]
  - 8.34 = log [H3O+]
  [H3O+] = 10-8.34 = 4.57 x 10-9 M
  On a calculator, calculate 10-8.34, or "inverse" log ( - 8.34).

 

[Cawolf]

pH = -log [H+]

(divide both sides by -1)

-pH = log[H+]

(raise both sides to the 10)

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

(10^logx = x)

10^(-pH) = [H+]

so, [H+] = 10^(-4.7)

approximate it to 10^-5 which is slightly larger than 0.00001

This is close enough for the MCAT as you know it is slightly larger than our answer, so 0.00002 is the best choice.

 

[heloothereits]

you should look at leah4sci.com videos on mcat math.