Final answer:
To calculate the salinity from sodium and chloride in water, you need to determine the formula mass of NaCl (58.44 amu), convert the mass percent of NaCl to grams per liter of water, and take into account that NaCl dissociates into two ions in solution.
Step-by-step explanation:
To calculate the salinity of water from sodium and chloride, we first need to determine the formula mass of sodium chloride (NaCl). The formula mass is obtained by adding the atomic masses of one sodium atom and one chlorine atom. From the periodic table, we find the atomic mass of sodium (Na) is 22.99 amu and for chlorine (Cl), it is 35.45 amu. Thus, the total formula mass for NaCl is 58.44 amu.
If we look at a practical example, such as seawater with approximately 3.0% NaCl by mass, we first convert this percentage to grams if we have 100 g of seawater, leading to 3 g of NaCl. To find the molarity, we would divide the mass of NaCl by its formula mass and then by the volume of water in liters. For molality calculations, which are often used to determine the colligative properties such as freezing point depression or boiling point elevation, we use the mass of the solvent in kilograms.
For instance, if you have to prepare a saturated NaCl solution at 0°C, you could dissolve about 36 g of NaCl in 100 g of water. To calculate the molality, you'd divide by the formula mass to get moles of NaCl, and then divide by the mass of the water in kilograms. Keep in mind that NaCl dissociates into two ions - Na+ and Cl- - this means that 1 mol of NaCl results in 2 mol of particles, affecting properties such as freezing point and vapor pressure of the solution.