1.Homework 1 Problem: G5

[MaryBanh_2K]

Problem: A student prepared a solution of sodium carbonate by adding 2.111 g of the solid to a 250.0-mL volumetric flask and adding water to the mark. Some of this solution was transferred to a buret. What volume of solution should the student transfer into a flask to obtain (a) 2.15 mmol Na; (b) 4.98 mmol CO32; (c) 50.0 mg Na2CO3?

I was working on this problem but got a little confused after comparing my answer with a classmate. For this problem, I found the molarity and used it to find the volume of solution. However, after getting Na2CO3’s molarity, 0.08 M, I was told to multiply it by 2 because there are 2 Na’s in Na2CO3. Why is this the case? Why don’t we triple the value if there are 3 O’s as well?

[Chem_Mod]

Once you find the molarity of Na2CO3, you can find the volume needed to produce x amount of Na+, CO3 2-, etc.

For part a, you are asked to find the volume to produce 2.15 mmol Na+. There are two mol of Na+ per mol of Na2CO3, as given by the subscript 2. Therefore, you would need to divide by 2, not multiply 2.

For part b, there is 1 mol CO3 2- per mol of Na2CO3 so you would not need to divide or multiply by anything.

[Alan Huang 1E]

You should double molarity in (a) because you are asked to find volume of Na, which has 2 moles.

You should NOT triple molarity in (b) because CO3 is one mole. Don't see it is 1 mole C + 3 moles O, but rather one mole of the entire CO3.