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What mass of NaOH in 0.5L of water can make a pH of 13?

Chemistry
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What mass of NaOH in 0.5L of water can make a pH of 13?

Nov 24th, 2014

We know that pH + pOH = 14

If pH = 13, then pOH = 14-13 = 1

pOH is defined as pOH = -log[OH]

We need to solve for the [OH]:

1 = -log[OH]

-1 = log[OH]

10^(-1) M = [OH]

0.1 M = [OH]

Now we have the molarity of the solution.  We need 0.1 M OH- in the solution.

Since NaOH is a strong base, it completely dissociates in water:

NaOH ---> Na+ + OH-.  There's no equilibrium here.  It's a complete dissociation.  So all of the NaOH in solution is in the form of OH-.

We case use the molarity and the volume to find moles of NaOH:

0.1 mol/L NaOH * (0.5L) = 0.05 moles NaOH needed

Now, use the molecular mass of NaOH to find the number of grams you need:

0.05 moles NaOH * (40 g/mole NaOH) = 2 g of NaOH needed

Nov 24th, 2014

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