What is the pericardial sac? what type of tissue makes up the sac?

Pericardial sac also known as pericardium is a cone-shaped sac made of fibrous tissue surrounding the heart and the blood vessels. It has two parts: the inner coat which is double-layered supporting the heart; and the outer coat which is solid and heavily built attached to the diaphragm and the rear part of the breastbone.

we would need to see the picture to answer it

the relative atomic mass of an element is the mass of one atom of that element, relative to 1/12th of the mass of a  1212c atom. to convert to an absolute mass in kg, it is necessary to multiply by  1.661×10−271.661×10−27.

the relative atomic mass is roughly equal to the number of neutrons, plus the number of protons, in the nucleus of the atom.

for elements with more than one stable isotope (different types of nucleus with different numbers of neutrons but the same number of protons), the quoted mass is usually the average mass, weighted by the relative abundance of the different isotopes.