1) A mixture weighting 8.75g contains only SiO, and NH,Cl, when this mixture was heated it's mass was
decreased to 3.50g. The mass % of NH,Cl in this mixture is : A ) 40% B) 67% C) 33% D ) 60%

From the given question you have:

Mass of mixture of SiO2 and NH4Cl is 8.75g

After heating the mixture,

Mass of mixture of SiO2 and NH4Cl is 3.50g

It is important to understand that when you heat the mixture of SiO2 and NH4Cl, SiO2 is a very stable material and doesn't undergo any decomposition whereas NH4Cl decomposes to give NH3 and HCl gas. Thus, upon heating you will be left with SiO2 only and all the NH4Cl will get converted to ammonia and HCl.

NH₄Cl ----> NH₃ + HCl

Thus, loss of mass corresponds to mass of NH4Cl

And the mass remaining corresponds to SiO2 in the compound.

Loss of mass = Original mixture mass - mass of heated mixture

Loss of mass = 8.75-3.50 = 5.25g

Percentage loss = [5.25/8.75] x 100 = 60%

Thus, % of NH4Cl in given mixture = 60%

Option D = 60%