Calcium chloride (CaCl) is a salt used for cheese making in two parts of the process: renneting, and salt brining. The standard is to use a 32% solution of calcium chloride dissolved in pure water, and when cheese making recipes call for the use of it that is what they expect.
When using pasteurized milk for cheese making, at least some calcium has likely precipitated out of solution from the milk and is no longer available to help rennet glue casein micelles together to form milk curds. Calcium chloride added to the milk before rennet will dissolve easily, adding calcium ions to help with curd formation. Typically the recommended rate is ¼ teaspoon of 32% CaCl solution to each gallon of milk.
Depending on the season, calcium chloride may be desirable to add to raw milk as well if the curds formed are weak.
When salting a cheese by using a saturated brine, one issue many cheese makers face is that the surface of the cheese will become slimy. This is because the concentration of calcium inside the cheese is higher than the brine, so calcium will be leached from the outer surface of the cheese into the brine, causing a loss of structure in the cheese surface so it feels soft and mushy. Adding some calcium chloride to the brine will bring the calcium concentrations closer to equilibrium and keep the cheese surface intact.
A similar issue shows up with the acidity of the cheese, where a little bit of white vinegar added to the brine keeps the acidity of the brine similar to the cheese to stop any leaching of lactic acid from the cheese.
To make a 32% calcium chloride solution, measure out an amount of ideally pure (reverse osmosis or distilled) water by weight in grams, and then measure out the corresponding weight of calcium chloride crystals to be dissolved. (These can be obtained most easily by buying pickle crisp used for canning, which is usually just pure calcium chloride.)
The formula for how much calcium chloride crystals to add is derived from the relationship between the weight of calcium chloride being 32% of the total weight of both water (Ww) and calcium chloride (Wcc):
Wcc / (Wcc+Ww) = 0.32
Solving for the weight of calcium chloride, we get:
Wcc = 0.32 x Ww / 0.68
This could be simplified further to an approximation:
Wcc = 0.4706 x Ww
As an example, if you have 20 grams of water, you would need approximately 0.32 x 20 / 0.68 = 9.4 grams of calcium chloride crystals to be added to it to get a 32% solution.
Beware, when you add calcium chloride to water, it will heat the water up (as it dissolves it generates an exothermic reaction). It shouldn't be a problem, but it is preferred to use a glass jar to mix them so that it won't cause any problems. This reaction, generating heat, is why calcium chloride crystals are used as ice melt; when the crystals come in contact with snow and ice, they dissolve into the snow and ice and heat it up, causing it to melt.