Chemistry and Chemical Reactivity (9th Edition)

x is the mass percentage of CaO. $2/1\times 7.695\ g\times x\div 56.08\ g/mol+ 2/1\times 7.695\ g\times (1-x)\div 40.30\ g/mol=0.125\ L\times 2.55\ M$ $0.274x+0.382(1-x)=0.319$ $x=0.5875=58.75\%$ MgO: $41.25\%$