# How many hydroxide ions are there in 3.5 grams of calcium hydroxide?

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To find out how many hydroxide ions are in 3.5 grams of calcium hydroxide you must first find out how many formula units of calcium hydroxide are present. Calcium hydroxide has the formula `Ca(OH)_2`

Start by finding the molar mass of calcium hydroxide:

40.1 g/mol for Ca + 2(16.0 g/mol) for O + 2(1.00 g/mol) for H = 74.1 g/mole

The number of moles of Ca(OH)2 is its mass multiplied by its molar mass:

3.5 grams x (1mol/74.1 g) = 0.047 moles

The number of molecules is moles multiplied by Avogadro's Number:

0.047 moles x (6.02 x 10^23 molecules/mole) = 2.83 x 10^22 molecules

Each calcium hydroxide formula unit contains 2 hydroxide ions, so the total number of hydroxide ions is:

2 x (2.83 x 10^22) = **5.66 x 10^22 hydroxide ions**