Here are the molar masses of these atoms:
Ra = 226 g/mol
H = 1.008 g/mol * 2 (there are two hydrogen atoms in radium hydroxide)
O = 15.999 g/mol * 2
The molar mass of radium hydroxide is therefore 260.014 g/mol.
Divide this number by 987 grams to cancel the units in order to obtain the number moles.
260.014 g/mol / 987 g = 0.263 mol
