How to calculate the percentage of calcium carbonate in sea shells in an experiment
$\ce{CaCO3}$ is basic so it will neutralize the acid.

$$\ce{CaCO3 + 2HCl -&gt; CaCl2 + H2O +CO2}$$

In your experiment you need to devise a way of finding out what the end point of the reaction is (using a $\mathrm{pH}$ indicator to find when the mixture is neutral). You need to be able to measure the amount of hydrochloric acid used in moles  (you should measure the volume and you know the concentration. The number of moles of water is twice the amount of moles of the calcium carbonate (from the chemical equation above).

You should measure the mass of the amount of shell you use at the start. You will be able to figure out the mass of the calcium carbonate in that sample of shells that you use (since you now know the number of moles of calcium carbonate). Then:

$$(\frac{\mathrm{Mass_{\ce{CaCO3}}}}{\mathrm{Mass_{shells}}}) \times 100\%=\mathrm{Percent}~\ce{CaCO3}$$

You should be able to do it with just one concentration but your teacher probably wants to talk to you about reliability and experimental errors.