how do i calculate this..

a solution of nitrous acid has a pH of 2.73 what is the [H3O+] ?

pH = -log(H3O^+)

The nitrous acid bit has nothing to do with the problem. ANY acid or base with a pH of 2.73 will have this H3O^+.

so is it -log^(2.73) ?

for some reason my calculator wont calculate it

Sure it will.

2.73 = -log(H3O^+)
-2.73 = log*H3O^+)
Hit the 10^x button and punch in -2.73.

To calculate the hydronium ion concentration, [H3O+], from the given pH value, you need to use the formula:

[H3O+] = 10^(-pH)

In this case, the given pH is 2.73. So, you can substitute the value into the formula and calculate as follows:

[H3O+] = 10^(-2.73)

To calculate this, you can either use a scientific calculator or simplify it using logarithmic properties. Here's how:

10^(-2.73) = 1 / 10^2.73 ≈ 0.001995

Therefore, the hydronium ion concentration, [H3O+], is approximately 0.001995.