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Amount of substances (HELP) watch

1. I am stuck on a question and I need help, please!
Question:
An impure sample of calcium carbonate (1g) reacted with 50cm^3 of 0.3M hydrochloric acid. The excess acid was titrated against 12.5cm^3 of 0.5M sodium hydroxide.
Calculate the percentage purity of the calcium carbonate.

Thank you in advance for the help guys!
2. Look up the theory of back titrations.
3. work out moles of sodium hydroxide
then using ratios, work out the moles of HCl, from this you can work out the amount of HCl that was excess. Take this away from 50cm3, then this will calculate he amount of HCl that reacted with the impure calcium carbonate, work out the moles reacted in ratio with calcium. Use mass/mr, to work out mass of calcium reacted, then thus caclluate the %

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