Answer:
The molar mass of Ca(OH)2 is:
Ca = 40.08 g/mol
O = 15.999 g/mol
H = 1.008 g/mol
Molar mass of Ca(OH)2 = (1 x 40.08) + (2 x 15.999) + (2 x 1.008) = 74.096 g/mol
To find the number of moles in 89.9 mg of Ca(OH)2, we can use the formula:
moles = mass / molar mass
moles = 0.0899 g / 74.096 g/mol = 0.0012122 mol
Therefore, there are 0.0012 moles (rounded to 4 decimal places) in 89.9 mg of Ca(OH)2.