calculate the hydronium ion concentration and the hydroxide ion
concentration in blood, the pH of which is 7.3 (slightly alkaline)

Hi teatea!!!

The pH of a solution is a measure of the molar concentration of
hydrogen ions in the solution and the numerical value is defined as
the negative base 10 logarithm of the molar concentration
(moles/Liter) of hydrogen ions:

pH = -log10[H+]  

then the molar concentration (moles/Liter) of hydrogen ions is:

[H+] = 1 x 10^(-pH) M 


The pOH of a solution is a measure of the molar concentration of
hydroxide ions in the solution and the numerical value is defined as
the negative base 10 logarithm of the molar concentration
(moles/Liter) of hydroxide ions:

pOH = -log10[OH-]

Then the molar concentration (moles/Liter) of hydroxide ions is:

[OH-] = 1 x 10^(-pOH) M

 
We also have the following relationship between hydronium ion molar
concentration and the hydroxide ion molar concentration in water
solutions:
pH + pOH = 14 


pH of blood  = 7.3 then pOH = 14 - 7.3 = 6.7

This means:

[H+] = 1 x 10^(-7.3) M = 0.0000000501187 M
[OH-] = 1 x 10^(-6.7) M = 0.000000199526 M

Hope this helps. If you need a clarification please post a request for
it before rate this answer.

Regards.
livioflores-ga