chem question help please

Question

If 2.6 g of sodium metal completely reacts with 24.1 mL of pure water (which is in excess), what is resulting molarity of the sodium hydroxide solution?

Answer 1

Hello, Molly,

I did this quickly, so please check my work closely.

We need a balanced reaction. I get:

2Na + 2H₂O = 2NaOH + H₂

I set up a table for calculating the moles of reactants and resulting products. My table won't fit in this space, so I'll try explaining it as best I can. Reply if you are uncertain about a certain step.

Water is in excess, so we'll assume all of the sodium reacts. (Don't try this at home, as they say). The balanced equations tells us we get 2 moles of NaOH for every 2 moles of Na, or a 1/1 mole ratio of the two. We'll conclude the 0.113 moles of Na forms 0.113 moles of NaOH.This is done with 24.1 ml of H₂O, which is also 24.1 grams. We need to subtract the moles of H₂O consumed in the reactio to find the amount remaining. Subtract the moles of Na consumed from the original moles of water (since the equation tells us we use 2 moles of H₂O for every mole of Na).

Convert the total moles of water to grams, which at a density of 1g/ml, is also the number of ml water. So we have 0.113 moles of NaOH in 22.085 mlwater. Molarity is defined as moles per liter, so we need to convert the ml into liters. Then simply divide moles NaOH by liters H₂O to get Molarity.

I did not pay attention to sig figs, but be aware there are only 2 in the question. The answer I get is thus 5.1M.

Bob