The molar mass of Ca(OH)2 is 74.09 g/mol.

Step 2: Calculate the number of moles of Ca(OH)2 needed to prepare 100 mL of a 0.1 M solution.

The number of moles of Ca(OH)2 needed can be calculated using the formula:

```

Moles of Ca(OH)2 = Concentration (in mol/L) × Volume of solution (in L)

```

In this case, the concentration is 0.1 M and the volume of the solution is 100 mL, which is equivalent to 0.1 L.

```

Moles of Ca(OH)2 = 0.1 mol/L × 0.1 L = 0.01 mol

```

Step 3: Calculate the mass of Ca(OH)2 needed.

The mass of Ca(OH)2 needed can be calculated by multiplying the number of moles by the molar mass.

```

Mass of Ca(OH)2 = Moles of Ca(OH)2 × Molar mass of Ca(OH)2

```

In this case, the number of moles is 0.01 mol and the molar mass of Ca(OH)2 is 74.09 g/mol.

```

Mass of Ca(OH)2 = 0.01 mol × 74.09 g/mol = 0.7409 g

```

Therefore, 0.7409 grams of Ca(OH)2 are needed to prepare 100 mL of a 0.1 M Ca(OH)2 solution.