What is the relationship between the International System of Units (SI) and the metric system?

Which of the following is the SI base unit for mass?

Convert 5 kilometers to meters.

If a reaction requires 2 moles of a substance, how many molecules are needed? (Use Avogadro's number: 6.022 x 10²³)

Combine the concepts of mass and volume to calculate the density of a substance given its mass in kilograms and volume in cubic meters.