For each of the reactions, calculate the mass (in grams) of the product that forms when 15.39 g of the underlined reactant completely reacts. Assume that there is more than enough of the other reactant. a. 2 K(s) + Cl2(g)-2 KCl(s) b. 2 K(s) + Br2(l)-2 KBr(s) c. 4 Cr(s) + 3 O2(g)-2 Cr2O3(s) d. 2 Sr(s) + O2(g)-2 SrO(s)

The mole concept is a fundamental principle in chemistry that provides a bridge between the microscopic world of atoms and molecules and the macroscopic world we can measure. A mole is defined as the amount of substance that contains as many entities (atoms, molecules, ions, etc.) as there are in 12g of carbon-12, which is approximately 6.022 x 10²³ entities and is known as Avogadro's number.

Imagine a huge collection of tiny, indistinguishable objects — that's essentially what a mole represents, with every mole containing the same number of particles, no matter what material you're measuring. For example, one mole of potassium (K) will contain exactly 6.022 x 10²³ atoms of potassium, just as one mole of water contains the exact number of water molecules.

This concept allows chemists to count particles by weighing them. Knowing how many moles of a substance you have can help you predict how much product you'll get in a chemical reaction, how much reactant you need, or the volume that a gas will occupy at a given temperature and pressure.

Molar mass, the mass of one mole of a substance, is a crucial value that links the mole concept to measurable quantities. You can find the molar mass of an element by looking at the standard atomic weight listed on the periodic table, expressed in grams per mole (g/mol).

For compounds like KBr or Cr₂O₃, you have to calculate the molar mass by adding up the molar masses of each element, multiplied by the number of times that element appears in the compound. For instance, the molar mass of KBr is found by adding the molar mass of potassium (K) and bromine (Br). So, if K is approximately 39.1 g/mol and Br is 79.9 g/mol, the molar mass of KBr will be 39.1 g/mol + 79.9 g/mol, which equals 119.0 g/mol.

Having the molar mass enables you to convert between the mass of a substance and the number of moles it represents. This calculation is often the first step in making other stoichiometric predictions about a chemical equation.

Stoichiometric calculations are the bread and butter of figuring out the different aspects of chemical reactions. Understanding the ratios of substances that react and are formed (stoichiometry) is vital. For example, if a balanced chemical equation tells you that 2 moles of potassium (K) react with 1 mole of chlorine gas (Cl₂) to produce 2 moles of potassium chloride (KCl), it's providing you the stoichiometric ratio of reactants to products which, in this case, is 1:1.

In a stoichiometric calculation, you typically start with a known mass or moles of a reactant, use the molar mass to convert mass to moles (if needed), then use the mole ratio from the balanced equation to find the corresponding moles of the product. Finally, you convert that amount back into mass using the molar mass of the product. This process requires careful attention to conversion factors and the coefficients of the reactants and products in the chemical equation.

By following these calculations with precision, chemists can predict exactly how much of a product can be made from a given amount of reactant, which is critical in fields ranging from industrial manufacturing to pharmacology.