M odule 1 scenario-based Problems
Sports Drink Formulations
A new range of “Thirstade” sports drinks is being developed. These are comprised of sugars and salts in powder form, designed to be made up with water. Trials were expected to proceed immanently, but a number of serious issues have arisen which call into question aspects of the formulation chemistry. It turns out that finding the right dose of salts is
not so straightforward.
Solubility Struggles (5 marks)
It is important that the powder formulations dissolve completely in water. One mixture was produced containing NaCl, CaCl2 and K3PO4 salts. Individually, each of the salts dissolves in water but if they are combined and added to water an insoluble precipitate forms between the calcium ions and the phosphate ions.
Q1. Write out a balanced chemical reaction for the precipitation, including the correct charges and the chemical state of each species.
Hint. As an example of how to approach this, consider mixing aluminium nitrate & potassium hydroxide solutions instead. The mixed solution contains Al3+, NO3–, K+, and OH– ions. According to solubility rules, nitrate ions form soluble salts with all cations including both Al3+ and K+ so no nitrate salt is expected to precipitate. Hydroxide ions form soluble salts with K+ but insoluble salts with most cations including Al3+, and so Al(OH)3 would be expected to precipitate as solid particles, giving the solution a milky appearance. Accounting for the charges and stoichiometric proportions, this reaction could be written as: Al3+(aq) + 3 OH–(aq) Al(OH)3(s).
In this example the remaining “spectator” ions (NO3–, K+) stay in solution and are not involved in the reaction, so there is no need to include them in the reaction.
ANSWER:
Q2(a) Phosphate ions in solution may be converted to hydrogen phosphate ions, HPO42-(aq), that are also capable of forming precipitates.
Write a balanced chemical reaction to explain how the aqueous phosphate ion can react with water molecules to produce aqueous hydrogen phosphate ions, HPO42-(aq). Include the charges and the chemical state of each species.
ANSWER:
Q2(b) Considering the type of reaction this is, what is the role of the reactant water molecule?
ANSWER:
Cation Catastrophe? (3.5 marks)
Some of the formulations include CaCl2. An observant worker noticed that one batch from the supplier appeared to have powder flakes with a different appearance and the judging from the volume of material in the container, had a density much greater than usual. Suspecting that it was not CaCl2 at all but a different salt, further investigation was carried out.
Analysis an aqueous solution made up from this material determined that 1.000 g of the solid contains 0.387 g of chloride (Cl–). Assume that the solid sample contains only an ionic salt of the form MCl2, where the unknown metal cation M accounts for the remaining mass. This data may be used to infer the identity of M as follows.
Q3(a) 0.387 g of chloride ions (Cl–) corresponds to how many moles? Show your working and enter your answer with units in the table below.
Q3(b) Consider what is the (stoichiometric) ratio of metal ions to chloride ions in the salt MCl2. Based on that and on your answer to Q3(a), how many mole of metal ions are present in 1.000 g of sample? Enter your answer with units in the table.
Q3(c) What is the molar mass of the metal ion?
hint – you need the number of mol from Q3b, along with the mass of the metal ions in a 1.000 g sample of MCl2. Enter your answer with units in the table.
Q3 Summary Table. Fill in all the shaded table cells.
Answers should have sufficient (3 or 4) significant figures and always include the units.
| quantity | working | Answer & units | |
| Q3(a) | n (Cl–) | ||
| Q3(b) | n (M2+) | ||
| Q3(c) | mass, m (M2+) | ||
| Molar mass of M2+ |
Q4. Consult a periodic table to decide which metal ion is present in the salt sample, based on its molar mass. It might not be calcium! Comment briefly on whether this salt is safe to ingest.
ANSWER:
Tonic type? (6 marks)
Either dehydration or hyperhydration from excess water intake may affect athletic performance and have serious health implications. Water will tend to flow through semi-permeable cell membranes into the region with higher concentration of solute (e.g. electrolyte), in a process known as osmosis. Sports drinks may contain different levels of electrolytes and carbohydrate depending on the phase of exercise [1]. They may be categorised based on their osmolarity (a kind of total concentration) relative to that of blood plasma (approximately 290 milli osmol/L):
- hypotonic drinks for use before training or competition (osmolarity less than 290 milli osmol/L)
- isotonic drinks during exercise (osmolarity ~290 milli osmol/L)
- slightly hypertonic drinks after exercise (osmolarity greater than 290 milli osmol/L)
The first step in determining osmolarity of a solution is to calculate the molar concentrations of all the solutes.
Q5. Complete the shaded cells of the table below to calculate the concentrations of sodium chloride and glucose (C6H12O6) if the following amounts of were dissolved in 400 mL of water. (Report concentration values to 4 decimal places).
| Substance | Mass, m /g | Molar mass, Mr / g mol-1 | Number of mol, n /mol | Concentration, c / mol L-1 |
| NaCl | 1.01 | |||
| Glucose (C6H12O6) | 11.80 |
Osmotic pressure is a colligative property – it depends on the concentrations of all the solute molecules or ions, but not on their identity.
In this case osmolarity, in osmoles per litre (osmol L-1) may be estimated by summing all the concentrations, including both Na+ and Cl– separately because NaCl is dissociated in solution. More details about osmolarity (beyond the scope of this assignment) are given in Appendix 4.
Q6(a). Complete the shaded cells of the table below to include the contributions to osmolarity from aqueous Na+ and Cl– ions and from glucose at the concentrations found in Q5. Add these three to calculate the total osmolarity concentration.
| Substance | Concentration, c / mol L-1 (from Q5) |
| Na+ (from NaCl) | |
| Cl– (from NaCl) | |
| glucose | |
| Total osmolarity concentration |
Q6(b). Based on the total osmolarity, would this formulation be categorised as hypotonic, isotonic or hypertonic? (hint: the comparison value of ~290 milliosmol L-1 has different units)
ANSWER:
Getting into Shape (9.5 marks)
The marketing department have drafted artwork showing pictures of some of the ions present in the drinks. Chemists have several different ways to represent chemical structures, but aspects of the depicted structures and shapes appear incorrect to the trained eye.
In the questions below, you should draw correct structures! You can create these in drawing software (even by inserting shapes in Word) or use pen and paper and take a photo / scan.
Q7. (a) Draw an electron dot structure for the oxygen molecule (O2).
(b) Draw an electron dot structure for the chloride anion (Cl–), accounting for the charge when considering the number of electrons.
ANSWER: insert image(s) here
Q8. Draw Lewis structures of the carbonate (CO32-), hydronium (H3O+) and sulfate (SO42-) ions. Be sure to account for all lone pairs and formal charges on atoms. Note that sulfur from row 3 of the periodic table exceeds the octet rule in sulfate.
ANSWER: insert image(s) here
Q9. Complete the shaded cells of the table below to work out the shapes of the carbonate, hydronium and sulfate ions according to VSEPR theory.
| Substance | CO32- | H3O+ | SO42- |
| a) Number of valence electrons on the central atom | |||
| b) Number of lone pairs on the central atom | |||
| c) Number of electron groups (domains) for the central atom | |||
| d) Electron Group Geometry | |||
| e) Molecular Shape |
Redox wreckage? (3 marks)
A formulation containing low concentrations of FeCl2 was trialled, on the basis that it could be marketed with the tag line “boost your iron levels”. However, once the powder was made up with water and exposed to the air, a brown sediment formed over time. Reaction of Fe2+(aq) with oxygen to produce a ferric (Fe3+) species is suspected.
One proposed reaction is
__ Fe2+(aq) + __ O2(aq) + __ H+(aq) __ Fe3+(aq) + __ H2O(l)
Q10(a) Write balanced half equations for the oxidation and reduction reactions, including electrons with their charge.
ANSWER
Oxidation reaction:
Reduction reaction:
Q10(b) Balance the overall reaction by inserting appropriate stoichiometric coefficients in front of the chemical species above.
References
[1] Urdampilleta A, Gómez-Zorita S, Soriano JM, Martínez-Sanz JM, Medina S, Gil-Izquierdo A, Hydration and chemical ingredients in sport drinks: food safety in the European context, Nutr. Hosp. 31(5), 1889–1899 (2015).
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