What name would be given to the salt formed when citric acid reacts with calcium carbonate?

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What is the formula for sulfamic acid and what is it used for?

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Why does water sometimes produce calcium carbonate(limescale) when it is heated? What is hard water?

Insoluble salts can be made using a precipitation reaction. The method can be used to find the formula of a salt. In an experiment, $6.0 {\mathrm{cm}}^3$ of a solution of the nitrate of metal X was placed in a narrow test tube and $1.0 {\mathrm{cm}}^3$ of aqueous sodium phosphate, ${\mathrm{Na}}_3{\mathrm{PO}}_4$ was added. The precipitate settled and its height was measured. The concentration of both solutions was $1.00 \mathrm{mol}/{\mathrm{dm}}^3$. 

The experiment was repeated using different volumes of the sodium phosphate solution. The results are shown on the graph.

What is the formula of the phosphate of metal X? Give your reasoning.