Hello, Robin,

The reaction follows this balanced equation:

H_{2}SO_{4} + 2NaOH = Na_{2}SO_{4} + 2H_{2}O

This tells us that it takes 1 mole of sulfuric acid to neutralize 2 moles of sodium hydroxide. If we can determine the moles H_{2}SO_{4} required for neutralization, that number would be doubled to determine the moles NaOH.

Find the moles H_{2}SO_{4} consumed by multiplying the 2L of H_{2}SO_{4} times it's molarity, 0.5M, which is defined as 0.5 moles/liter.

(2~~L~~) * (0.5 moles/~~L~~) = 1 mole H_{2}SO_{4}

Multiply by 2 to find the moles NaOH. That means 2 moles NaOH were present in the 3 L of the solution.

Find the concentration by division:

2moles NaOH/3 L = 0.66 M

Bob