Calculate the number of moles of calcium ions present in a 50.00 mL water sample that has a hardness of 75.0 ppm (hardness due to CaCO3)

Hardness = 75ppm = 75 mg / L

volume = 50 mL = 0.05 L

So, applying hardness formula:

Hardness = mass / volume

so, mass = hardness x volume = 75 x 0.05 = 3.75 mg = 0.00375 g

So, moles of CaCO3 = moles of Ca2 + ions = mass / molar mass of CaCO3 = 0.00375 / 100.06 = 0.00003747751 moles