是咁的
有題 percentage by mass 唔識計.

In an analysis of a sample of duralumin (an alloy of Mg and Al), 2.50g of the alloy is burnt in excess O2 and 4.68g of a white residue is produced.

Calculate the percentage by mass of Mg in the alloy.

+
已知 ans 為 7.32%

首先寫equation:
2Mg+O2->2MgO
4Al+3O2->2Al2O3

Let x mol, y mol be the number of mole of Mg and Al respectively.
我地就可以由MOLAR MASS(periodic table有比)obtain到兩條equation

24.3x+27y=2.5------------因為燒之前係2.5g
x*(24.3+16)+0.5y*(27*2+16*3)=4.68------燒之後4.68g, 而1 mole Al只係出到0.5 mole Al2O3

跟住...搵到mole之後自己計番mass ratio

利申: chem 攞3