What is the molar mass of water??

Each water molecule contains two hydrogen atoms ($M = \text{1.0079 g/mol}$) and one oxygen atom ($M = \text{15.999 g/mol}$). Therefore, the molar mass is:
${M}_{{H}_{2} O} = 2 \times \text{1.0079 g/mol" + "15.999 g/mol" = "18.015 g/mol}$