# Chemistry

posted by .

At 40 degrees Celsius, the value of Kw is 2.92 X 10^-14

a.) calculate the [H+] and [OH-] in pure water at 40 degrees celsius

b.)what is the pH in pure water at 40 degrees celsius

c.)if [OH-] is .18M , what is the pH

• Chemistry -

H2O ==> H^+ + OH^-
You know Kw = (H^+)(OH^-) = 2.92 x 10^-14.
AND you know (H^+) = (OH^-)in pure water.
Then pH = - log (H^+)
Post your work if you get stuck.

• Chemistry -

I already worked out (c), but the rest is still just mush. I'm sorry for my stupidity, but this just doesn't make sense. I can find [H+] or [OH-] as long as I have the concentration of 1 of them, so, ???

• Chemistry -

By the way, how do you find H+ and OH- concentrations from the pH? I have scoured my text book, but found nothing.

• Chemistry -

(a)
H2O ==> H^+ + OH^-
for every x mols H2O that dissociate, there are x mols H^+ and x mols OH^- so
since Kw = (H^+)(OH^-) = 2.92 x 10^-14
then x*x= 2.92 x 10^-14
so x = sqrt 2.92 x 10^-14

(b)I assume you can do pH = -log(H^+) now that you know (H^+).
The answer you should obtain is 6.76731 which rounds to 6.77

Here is how you find pH from (H^+).
Suppose pH = 5.32
pH = - log(H^+)
5.32 = -log(H^+)
-5.32 = log(H^+)
So you take the antilog of -5.32. To do that, enter 5.32 on your calculator, change the sign to - (or enter -5.32 at the beginning), then punch the 10x key on your calculator. If you have done it right, you should get 4.7863 x 10^-6. Of course that's too many significant figures; however, I copied ALL of the digits so you can check your calculator ability. Now just to make sure you get it, use pH = 6.76731 and see if you get the same answer as you have for (a).
Let me know if this isn't clear.

## Similar Questions

1. ### chemistry

What mass of CaCl2(s) must be dissolved in pure water at 10 degrees celsius to make a 26.3 mL solution and to increase the solution temperature to 16.4 degrees celcius?
2. ### Chemistry

A 5.0-g Sample of KBr at 25.0 degrees celsius dissolves in 25.0 degrees celsius 25.0 mL of water also at 25.0 degrees celsius. The final equilibrium temperature of the resulting soltution is 18.1 degrees celsius. What is the enthalpy …
3. ### Chemistry

Calculate the joules of energy involved when 11 grams of water at 10.0 degrees Celsius cools and then freezes at 0.0 degrees Celsius. How many kilocalories are required to raise the temperature of 305 gtams silver from 30 degrees Celsius …
4. ### Chemistry

A 100g sample of water at 90 degrees celsius is added to a 100g sample of water at 10 degrees celsius. The final temperature of the water is which of the following: a) between 50 degrees celsius and 90 degrees celsius b) 50 degrees …
5. ### chemistry

Calculate the enthalpy change for converting 10.0g of ice at -25 degrees Celsius to water at 80 degrees Celsius. The specific heats of ice, water, and steam are 2.09j/g degrees Celsius, 4.18j/g degrees Celsius, and 1.85j/g degrees …
6. ### Chemistry

When 22.7mL of o.500 M H2SO4 is added to 22.7mL of 1.00 M KOH in a coffee-cup calorimeter at 23.50 degrees Celsius, the temperature rises to 30.17 degrees Celsius. Calculate the delta h of this reaction. (Assume that the total volume …
7. ### chemistry

a 225.0 g sample of an unknown, silver, pure metal was heated to 95.5 degrees celsius then put into 150.50 g of water at 22.5 degrees celsius. the water was heated by the hot metal to a temperature of 31.4 degrees celsius. what is …
8. ### Physics 20

1. How much heat energy is required to change 500 grams of water at 100 degrees Celsius to steam at 100 degrees Celsius?
9. ### Physics

Please Help! I am having difficulties grasping these concepts. Explain please! 1. How much heat energy is required to change 500 grams of water at 100 degrees Celsius to steam at 100 degrees Celsius?
10. ### Physics

The water from two buckets is mixed together. One bucket contains 5kg of water at 20 degrees Celsius and the other contains 1kg of water at 80 degrees Celsius. What is the final temperature of the mixture, assuming no heat is lost …

More Similar Questions