We are given with the mass of pure iron that reacts with oxygen to form an oxide which has a given mass as well. the mass of oxygen reacted is 8.15-6.25 g or 1.9 grams. THen we convert the mass of the reactants to moles. Iron is equal to 0.1119 moles and oxygen is equal to 0.1188. We divide each number to the less amount. Hence iron is 1 and oxygen is approx 1. The empirical formula hence is FeO or ferrous oxide or Iron (II) oxide.
The rate constant for 1st order reaction is
K = (2.303 /t) log (A0 /A)
Where, k is rate constant
t is time in sec
A0 is initial concentration
(6.82 * 10-3) * 240 = log (0.02 /A)
1.63 = log (0.02 /A)
-1.69 – log A = 1.63
Log A = - 0.069
A = 0.82
Hence, 0.82 mol of A remain after 4 minutes.
Answer: 35.4 g
Explanation:
The balanced reaction is :

To calculate the moles :


According to stoichiometry :
2 moles of phosphorous chloride are produced by = 3 moles of 
Thus 0.37 moles of phosphorous chloride are produced by=
of 
Mass of 
Thus 35.4 g of chlorine reacted with the phosphorus
Compound sample: In a compound there remains bond between two or more atoms, which is produced by a chemical reaction. As for example:
. Thus water is a compound.
Compound homogeneous mixture: In a material which is produced more than one element but both remains in the same phase. There will be uniformity in the mixture. Like gas with gas, liquid with liquid etc. Example: Air which is mixture of oxygen, nitrogen, carbon di oxide where all the compounds are in gaseous form.
Compound Heterogeneous mixture: The mixture where the compound or elements by which the mixture is prepared are in different phase. Like chalk (CaC
in water (
(liquid), which is a heterogeneous mixture.