Question

Bridget, a Chemistry of the Sea IA, is sitting at home on a Friday night and decides to entertain herself by making synthetic seawater with chemicals she has around the house (being a chemical oceanographer, she has a wide range of available household chemicals). She wants to make a solution containing the six most abundant ions in close approximation to their actual seawater levels. Her plan is to weigh the chemicals into a vat and then add distilled water until the final weight of the solution is 1 kg. Her goal is to have the following concentrations: [Ion] (mmol/kg) Na+ (470) Mg2+ (50) Ca2+ (10) K+ (10) Cl‐ (545) SO4 2‐ (30) She has the following chemicals: NaCl(s) (common flavor additive) MgSO4(s) (used in fertilizer, explosives, foot soaks) CaCl2 (s) (drying agent, preservative) HCl(s) (stolen from lab – pretend it’s a solid) KOH(s) (lye, for making soap) Mg(OH)2(s) (laxative) Assume all the ions dissociate completely (don’t worry about the H and OH– they will combine to make water in your solution and the excess will determine the pH). How many grams of each chemical should she add to get the correct ionic concentrations?

Answer 1

Name of Ion	Concentration in mmol/Kg	mols/Kg	Molecular weight of these salts	g/kg	Toal Weight in gm to be mix to get desired ionic concentration per kg
Na+	470	0.470	58.44 g/mol	27.466 g/Kg	494.3 g
Mg2+	50	0.05	58.3197 g/mol	2.915 g/Kg	52.47 g
Ca2+	10	0.01	110.98 g/mol	1.1098 g/Kg	19.97 g
K+	10	0.01	56.1056 g/mol	0.5610 g/Kg	10.09 g
Cl-	545	0.545	36.46094 g/mol	19.87 g/Kg	357.66 g
SO42-	30	0.03	120.366 g/mol	3.61 g/Kg	64.98 g

27.466 X + 2.915X + 1.1098 X + 0.5610X + 19.87X + 3.61X = 1000g

55.53X= 1000 g

X = 18

Multiply by factor 18 we calculated above to all the weight in the second last column of above table to get exact weight in gram of salt should be mixed to get desired ionic concentration mentioned in the question.