Concentration is the level of a substance in a mixture, expressed in various units. Molarity is weight per unit volume. The molar concentration of a substance is the number of moles of that substance in one liter of solution. The atomic weight of sodium and chlorine can be found on the periodic table. The molecular weight of NaCl is 58.44 g/mol. The molar concentration of a solution can be calculated using the formula (mass/molecular weight) ÷ (volume in liters). The molar concentration of a solution remains the same even if other substances are present. The molecular weight of magnesium bromide is 184.11 g/mol. The molar concentration of magnesium bromide can be calculated using the same formula. The molar concentration of a solution is in terms of “per liter of solution”, not “per liter of water”.
In chemistry, concentration is the level of a substance in a mixture of substances, such as the amount of sodium chloride found in the sea. Concentration can be expressed as various units, often given in terms of weights and volumes. Molarity is a form of weight per unit volume. The molar concentration of a particular substance is the number of moles of that substance dissolved in one liter of solution, regardless of how many other substances may be dissolved in that same solution.
In sodium chloride (NaCl), ordinary table salt, the atomic weight of the two substances – sodium and chlorine – can be found by referring to the periodic table. The atomic weight of sodium is 22.99. The atomic weight of chlorine is 35.45. This means that sodium chloride – an atom of both of these elements combined – has a molecular weight of 58.44. Since one mole of a substance is defined as its molecular weight in grams, one mole of NaCl is 58.44 grams (g).
As an illustration, if 537 milliliters (ml) of a solution contains 15.69 g of sodium chloride, but no other substances, the molar concentration of that solution is (15.69 g / 58.44 g) ÷ (537 ml / 1000 ml) = 0.50. The solution is 0.50 M in sodium chloride. If the solution contains another component, such as magnesium bromide, this solution remains 0.50 M in sodium chloride. It also has, however, a molar concentration of magnesium bromide.
The atomic weight of magnesium is 24.31. The atomic weight of bromine is 79.90. However, the molecular weight of magnesium bromide is not 24.31 + 79.90 = 104.21. This is because magnesium bromide has the chemical formula, MgBr2, as the valence of magnesium is +2, while the valence of bromine is only -1. Correctly, the molecular weight of magnesium bromide is 24.31 + (2 × 79.90) = 184.11.
If 24.72 g of magnesium bromide are present, the molar concentration of magnesium bromide is (24.72 g / 184.11 g) ÷ (537 ml / 1000 ml) = 0.25 M. This means that the solution is both 0.50 M in NaCl and 0.25 M in MgBr2. Interestingly, despite the decrease in water molecules in this second solution compared to the first – the concentrations are in terms of “per liter of solution”, not “per liter of water” – the molar concentration of sodium chloride it is the same for both. It is theoretically possible for an immensely large number of substances to be present in a single liter of solution, resulting in a collection of rather low molar concentrations with almost no water present.
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