Concentration by mass (C) It indicates the amount of solute by mass per unit of volume of solution : Csolute = mass volume solute solution In the SI, it is measured in kg/m3. However, it is usually measured in g/cm3, g/L or μg/L. Molarity (M) or molar concentration It indicates the amount of solute (in moles) per litre of solution : M = n L solute solution This way of expressing concentration is the most suitable when substances are involved in chemical reactions. 32 A can of soft drink contains 330 mL of liquid that has about 35 g of sugar dissolved in it. a) What is the concentration in g/L? b) The WHO recommends that children under 13 years of age should not consume more than 16 g of sugar per day, and those over that age should consume no more than 25 g. If you were only consuming sugar from that drink, what is the maximum amount of soft drink the WHO recommends that you drink per day? 33 In blood tests, a blood glucose level between 70-105 mg/dL is considered normal. If in a blood sample, 17 mg of glucose is found in 20 mL of blood: a) Is this within the normal range? b) Express the concentration in g/L. 34 Magnesium hydroxide (Mg(OH)2) is used to make antacid medicine. A typical commercial antacid contains 200 mg/mL. a) Calculate the molar concentration of Mg(OH)2. b) The instructions for use indicate that the maximum dose for an adult is 12.5 mL/day. How many moles of magnesium hydroxide can be taken at the most? 35 A glucometer indicates that a person's blood glucose level is 5.3 mmol/L. a) Calculate its molarity. b) The formula of glucose is C6H12O6. Express this concentration in g/L and mg/dL. A C T I V I T I E S 8. Solutions SOLVED PROBLEM 9 Calculate the concentration in g/L of a solution prepared by dissolving 5 g of salt in water until a volume of 250 mL is reached. Csolute = mass volume solute solution Substitute the data and solve: Csolute 5 g 250 mL 0.02 g mL = = =20 g L SOLVED PROBLEM 10 Calculate the molar concentration of a solution that is prepared by dissolving 5 g of salt in water until a volume of 250 mL is reached. • To find the moles of solute, calculate the molar mass of the solute. Find the atomic masses in the periodic table. M (NaCℓ) = 22.99 + 35.45 = 58.44 g/mol 5 g of NaCℓ × 1 mol of NaCl 58.44 g of NaCl = 0.086 mol of NaCℓ • Substitute the data and solve: M n L = = = solute soluti n 0.086 mol 0.25 L o 0.34 mol L 26
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