What is the concentration of [Ba ^2+] in a solution prepared by dissolving .210 g of Ba(OH)2 in enough water to make 450. mL of solution?

Question

matt101 · Answer

Concentration can be expressed in a number of ways. I'll use molarity (moles of solute per litre of solvent) since that's pretty common.

First find moles of solute. Ba(OH)2 is ionic and so dissociates into Ba2+ and 2OH- in water. However, since each molecule of Ba(OH)2 contains 1 Ba atom, the moles of Ba2+ produced is same as the moles of Ba(OH)2 that is dissolved (the moles of OH- however is twice that of Ba(OH)2). Find moles of Ba(OH)2 by dividing the mass by the molar mass: 0.21/171 = 0.00123 moles. Therefore there are also 0.00123 moles of Ba2+ present.

The concentration is moles of solute per litre of solvent, so in this case the concentration would be 0.00123/0.45 = 0.00273 M (molar).