Răspuns:
MMg(OH)2=24+2×16+2×1=58g/mol
n=m/M
m=3x58=174gMg(OH)2
MMg=24g/mol
24g.........................58g
Mg + 2H2O => Mg(OH)2 + H2
xg.............................174g
x= 174x24/58=72gMg
100g Mg.............90gMg pur
72gMg...................ygMg pur
y=72×90/100=64,8g Mg pur.