Asample of lemon juice is found to have a ph of 2.3. what is the concentration of hydrogen ions in the lemon juice?

It's equal to 10^(-2.3), or 0.00501 M, or 5.01 * 10^-3 moles/Liter

Explanation:

Well, pH = - log[H+]

Or, in words, pH is equal to -1 multiplied by the logarithm (base 10) of the hydrogen ion concentration.   So you have 2.3 = -log[H+].    We want to isolate the H+, so let's start simplifying the right hand side of the equation. First, we multiply both sides by -1.   -2.3=log[H+]   Now, the definition of a logarithm says that if the log (base 10) of [H+] is -2.3, then 10 raised to the -2.3 power is [H+]   So on each side of the equation, we raise 10 to the power of that side of the equation.   10^(-2.3) = 10^(log[H+])   and because 10^log cancels out...   10^(-2.3) = [H+]   Now we've solved for [H+], the hydrogen ion concentration!

some sources quote the lethal dose, for an average 170lbs opiate-naive individual is between 75 and 375 mg

it's all four options.