Chemistry question, help

1) What is the mass of 7.7 L of gasoline at 20 degress Celsius? Assuming the density of gasoline to be 0.68 g/cm^3

2) How many neutrons does an atom of each isotope contain?
a) Kr (b) Br (c) Os (d) Re

1) Mass = Volume * Density
= 7.7L * (0.68g/cm^3 * 1000cm^3/L) * (1kg / 1000g)
Mass = 5.2kg

2) Each element has several isotopes. Specify the isotopes.