Дано:
moles of magnesium and magnesium phosphide = 0.7 mol
mass of salt formed = 85.5 g

Найдем массу магния и массу фосфида магния в смеси:
Mg + Mg3P2 -> 2MgCl2

molar mass of Mg = 24 g/mol
molar mass of Mg3P2 = 24 + 2*31 = 86 g/mol
Let the mass of magnesium be x g, then the mass of Mg3P2 will be (85.5-x) g.

According to the given data:
x/24 = (85.5-x)/86
Solving the above equation, we find:
x = 12.5 g
Mass of Mg3P2 = 85.5 - 12.5 = 73 g

Найдем количество молей продукта образованной соли:
According to the balanced chemical equation:
1 mol of Mg + 1 mol of Mg3P2 produces 2 mol of MgCl2
0.7 mol of Mg + (73/86) mol of Mg3P2 will produce (2*0.7) mol of MgCl2

moles of MgCl2 formed = 1.4 mol

Найдем молярную массу продукта MgCl2:
molar mass of MgCl2 = 24 + 2*35.5 = 95 g/mol

Найдем массу образованной смеси газов:
mass of MgCl2 = 1.4 * 95 = 133 g

Найдем массу водорода в образованной смеси газов:
In MgCl2, the molar ratio of Mg to Cl is 1:2. So, the molar mass of hydrogen to chloride is 1:2. So, hydrogen is 2/3 of the total mass of MgCl2.
mass of hydrogen = (2/3) * 133 = 88.67 g

Найдем плотность образованной смеси газов относительно водорода:
As the volume of gases is proportional to the number of moles, the density of the gas mixture is the sum of the density of each gas times its volume fraction. Since hydrogen has a density of 0.0899 g/L at STP, the density of the gas mixture relative to hydrogen would be 0.0899 g/L.