Answer:
A . 6.3 In a healthy pond, the temperature is 16°C (61°F). What is the most likely pH of this pond
1) Molecular formula of ammonium sulfide
(NH4)2 S
2) That means that there are 2*4 = 8 atoms of hydrogen in each molecule of ammoium sulfide, so in 5.20 mol of molecules will be 8 * 5.20 mol = 41.6 moles of atoms of hydrogen
3) To pass to number of atoms multiply by Avogadro's number: 6.022 * 10^23
41.6 moles * 6.022 * 10^23 atoms / mol = 250.5 * 10^23 = 2.50 * 10^25 atoms
Answer: 2.50 * 10^25
Lithium's atomic weight is 6.94 g/mol.

Need anything else?
Answer:
Conservation of mass can be checked in an experiment . There are three steps to do it in a best way:
1. Weigh all the equipment and materials required in the experiment before the experiment.
2. Avoid spillage and evaporation during the experiment.
3. Weigh all the equipment and materials after the experiment.
If the mass is conserved then weight from step 1 is equal to weight from step
Alloy: alloy, metallic substance composed of two or more elements, as either a compound or a solution. The components of alloys are ordinarily themselves metals, though carbon, a nonmetal, is an essential constituent of steel. ... See also amalgam; ferroalloy; intermetallic compound
Ceramic: A ceramic is any of the various hard, brittle, heat-resistant and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcelain, and brick.
Synthetic polymer: Synthetic polymers are derived from petroleum oil, and made by scientists and engineers. Examples of synthetic polymers include nylon, polyethylene, polyester, Teflon, and epoxy. ... Vulcanized rubber is a synthetic (man-made) polymer, while pectin is an example of a natural polymer
That’s a *hint*