ANS:D - 40

NaOH (sodium hydroxide) contains 40% oxygen by mass. To find this, you need to calculate the molar mass of NaOH: Molar mass of NaOH=Atomic mass of Na+Atomic mass of O+Atomic mass of HMolar mass of NaOH=Atomic mass of Na+Atomic mass of O+Atomic mass of H =22.99 g/mol+15.999 g/mol+1.008 g/mol=22.99g/mol+15.999g/mol+1.008g/mol =39.997 g/mol=39.997g/mol The molar mass of oxygen in NaOH is 15.999 g/mol. Now, to find the percent of oxygen in NaOH: Percent of oxygen=Molar mass of oxygenMolar mass of NaOH×100Percent of oxygen=Molar mass of NaOHMolar mass of oxygen​×100 =15.999 g/mol39.997 g/mol×100=39.997g/mol15.999g/mol​×100 ≈40%≈40% So, NaOH contains approximately 40% oxygen by mass. Therefore, the correct answer is 40.