titration question

[tipton12]

A 0.4793g sample of primary standard grade Na2CO3 was treated with 40.00mL of dilute HCl. The solution was boiled to remove CO2 and then excess HCl was back-titrated with 8.70mL of NaOH. In a separate experiment, it was established that 27.43mL of the HCl neutralized the hydroxide in a 25.00mL portion of the NaOH solution. Calculate the molarities of the HCl and the NaOH solutions.

I need a starting point or steps. I was thinking about finding the number of moles of the standard Na2CO3 present. But other than that, I'm a little overwhelmed with what to do with all the volumes given. Please help on the homework problem!

[chem_tr]

Hello, I see that a lot of people encounter some difficulty in understanding these.

Please remember that molarity has the dimension of \frac{mol}{L} or \frac{mmol}{mL}, while multiplying this with volume gives mol or mmol, respectively.

You can try writing the reaction formula to see how many mols of reactants are required for a complete reaction, and you can find how many moles of HCl are in excess.

A last hint: if someone says that there is a primary standard, you can understand that its purity can be considered as 100%.