convert 12.64 g of NaOH to moles
.316 moles just divide by the molar mass of naoh which is 39.997. So 12.64 divived by 39.997 is .3160237. that is 3.16x10-1
molar mass of NaOH =22,990g/mol+15,999g/mol+1,001g/mol =39,99g/mol
n= mass/molar mass
moles of NaOH= 12,64 g * 1 mol NaOH /39,99g = 0,316 moles of NaOH in 12,64 g of NaOH