The average the natural abundances of the various isotopes of carbon to arrive at the fractional mass.
There are several information's already given in the question. Based on those information's, the answer can be easily deduced.
Amount of gasoline required by Harry's car to travel 25 miles = 1 gallon
Then
amount of gasoline required
by Harry's car to travel 15000 miles = 15000/25
= 600 gallons
So
Amount of CO2 released by burning 1 gallon of gasoline = 20 pounds
Then
Amount of CO2 released
by burning 600 gallon of gasoline = 600 * 20
= 12000 pounds
From the above deduction, it can be concluded that the amount of CO2 that will be added by Harry's car to the atmosphere is 12000 pounds.
Answer: -
The experiment Niven is doing is burning of Mg.
The first step would be finding the molar mass of MgO
Atomic mass of Mg = 24 g
Atomic mass of Oxygen = 16 g
Molar mass of MgO = 24 x 1 + 16 x 1 = 40 g
The balanced chemical equation for this reaction is
2 Mg + O2 -- > 2MgO
From the balanced equation we see that
2 Mg gives 2 MgO
2 x24 g of Mg O gives 2 x 40 g of MgO.
28g of MgO gives

= 46.66 g of MgO.
Nuclear decay or radioactive decay is a process by which the nucleus of an unstable atom loses energy that is in terms of its mass. The radioactivity may cause an atom to lose whichever of its subatomic particles.
The explanation as why some of the atoms go through nuclear decay while others do not its because of the stability of some atoms. Atoms usually tend to follow octet rule, those which do not follow through this may experience the nuclear decay.
Answer:
Ne
Explanation:
Noble gas, Unreactive therefore does not exist as a diatomic molecule