Textbook Q 6A.21

[Ivy Nguyen 3I]

"The value of Kw for water at body temperature (37 °C) is 2.1x10^-14. (a) What is the molar concentration of H3O+ ions at 37 °C? (b) What is the molar concentration of OH- in neutral water at 37 °C?"

The answer key shows that you solve this by setting 2.1x10^-14 = [H3O+][OH-] = x^2 and I was wondering why you can assume that H3O+ and OH- concentrations are equal in this case? Cant the two be different values and still equal that given Kw instead?

[Matthew Nguyen 2F]

I believe this is because we assume that H30+ and OH- concentrations are equal in neutral water regardless of what temperature it is.

[rachelcameron2E]

Yes, pure water has equal concentrations (1x10^-7) of hydronium and hydroxide ions, that's why we can calculate the Kw of pure water. If there were solutes in this problem, then you would need to know the hydroxide or hydronium concentration to solve for the other.

[Joellen 1B]

You can assume [H3O+] and [OH-] are equal because water is neutral.What I did was I found Ka and Kb using the given kw as you described you saw in the answer key.

[briana lay 3c]

Hello!
You can assume [H3O+] is roughly equal to [OH-] because water is a neutral substance. If there are different values for [H3O+] or [OH-], the water would no longer be a neutral substance. Hope it helps!

[Amanda Nguyen dis 2E]

Since water is a neutral substance you can make the assumption that [H3O+] = [OH-]. Hope that helps!

[Tony Chen 1F]

Since it's water, [H3O+] = [OH-] and pH = pOH. To add on, the pH of water changes as temperature changes. In other words, the pH of water will not be 7 as temperature changes. However, water is still neutral because [H3O+] = [OH-] and pH = pOH. So what makes water neutral is the equal concentration of hydronium and hydroxide ions, not pH=7. pH=7 is only true for water at 25 C

[Ruirui Lan]

Hi! I think this is because in neutral water, for every molecule of H2O used, one hydroxonium ion and one hydroxide ion forms. Therefore, in neutral water, the concentration of H3o+ and OH- should always be the same, and we can also infer that ph=poh. however, its also important to note that although ph can change with the change in temperature, this wouldn't mean the water would become more or less acidic, since it only depends on the relative amount of H3O and OH ions, which in this case remain the same.