Chemistry mole question

Question

Suppose 0.83lb of sodium dichromate is dissolved in a bucket containing 3567mL of water.

a) What is the resulting concentration (in mol/L) of the sodium dichromate solution?

b) How many moles of chromium atoms are in 15.0mL of the solution?

Answer 1

Hello, Yoon,

We need to convert the lbs into g. 1lb = 453.6g, so we have 376.5 grams of Na₂CrO₄. Find the molecular weight of sodium chromate, which I calculate to be 262 g/mole. By dividing the mass by the molar mass, we get the number of moles (= 1.44 moles Na₂CrO₄). This is dissolved in 3567 ml water. Convert this to Liters (=3.567 L) and then divide that into the moles to gives us moles/Liter, or Molar. I get 0.403M.

If we take 15 ml of a 0.403 M solution, we can calculate the number of moles of the solute. But note that Molar is (moles/liter), so we need to convert to 15ml into Liters (=0.015). Multiply the two (M x Vol in L) to give the moles present in 15ml (= 0.00604 moles). Since one mole = 6.022x10²³, multiply this number times the number of moles to get the number of Na₂CrO₄ molecules.

Be careful with sig figs. I see only two in the 0.83 number.

Bob