Based on this, which of the reactions below are double-replacement reactions?
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)
2 AgNO3(aq) + Zn(8) + 2Ag(s) + Zn(NO3)2(aq)
Pb(NO3)2(aq) + 2KI(aq) + 2KNO3(aq) + Pb12(s)
BaCl2(aq) + Na2SO4 (aq) + BaSO4(s) + 2NaCl(aq)

BaCl2(aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl(aq)

Explanation:

Chemical reactions are of different types namely; combination reaction, displacement reaction, double displacement reaction etc. Double displacement/replacement reaction is that reaction in which an exchange of ions occur between two reacting ionic compounds to form new products.

In this chemical reaction/equation;

BaCl2(aq) + Na2SO4 (aq) → BaSO4(s) + 2NaCl(aq)

2Cl- and SO42- are the ions exchanged in this reaction. Barium (Ba) displaces SO42-, while Sodium (Na) displaces 2Cl-, hence it is called DOUBLE DISPLACEMENT OR REPLACEMENT because the displacement involves two compounds/ions.

the ideal gas law equation is as follows

pv = nrt

where p - pressure

v - volume

n - number of moles

r - universal gas constant

t - temperature

so if the pressure, volume and temperature are already known

we are left with n and r

since r is the universal gas constant that has a known fixed value then r too is known

so we are left with 'n'

once we know temperature volume and pressure

we can find the number of moles of gas present using the ideal gas law equation

this is not my own answer, this is an answer for the same question by the user blahblahmali. i do not claim ownership of this account.