How To Find Percentage Purity Of Calcium Carbonate

Hey guys, i did an experiment on back titrations in school and im kind of confused as to how to work out the purity of my limestone sample step by step.As i seem to keep getting different purities.A sample of impure limestone was given to determine the percentage purity of calcium carbonate using back titrations.Approximately 4.5g of limestone was weighed in to a beaker and was dissolved.