S O LV E D P R O B L E M 1 4 A 20 % glucose infusion is a solution of sugar in water with a density of 1.15 g/cm3. We want to prepare half a litre of 20 % glucose infusion. Calculate the amount of sugar needed. Solution: 20 % glucose infusion d = 1.15 g/cm3 V = 1500 mL Solute: sugar Solvent: water Using the density of the solution, calculate the mass equivalent to 500 mL: d m v m d V = → = ⋅ = ⋅ = 1.15 g 500 575 g mL mL Use the richness of sugar in the glucose infusion as a conversion factor: 575 20 10 g of infusion g of sugar g of infusion ⋅ = 115 g of sugar 60 To prepare half a litre of a solution of sodium chloride (NaCℓ) in water at 5 % by mass, how much sodium chloride will you need? Data: dsolution = 1.05 g/cm3. 61 We prepare a solution by mixing 20 g of sodium hydroxide in 200 mL of water. The density of the solution is 113 g/mL. Calculate the concentration in percentage by mass, in g/L, and molarity (M). 1 62 Analyse the news article. What exactly i s homeopathy? Homeopathy is based on a doctrine developed by the German physician Samuel Hahnemann at the end of the 18th centur y. It is based on the principle that " like cures like". According to it, the same substance that causes symptoms of a disease must be used to cure it, diluted in water in infinitesimal proportions. For example, caf feine wakes us up, but if taken in tiny proportions it does the opposite: it makes us sleepy. A homeopathic preparation for sleeping can be obtained by mixing one part of caf feine with 99 parts of water. It is shaken and the solution is mixed with another 99 parts of water. And so on . These preparations are usually taken in the form of sucrose and water tablets. Sometimes, no traces of the active ingredient to be administered remain in the tablet. The scientific evidence against the ef fectiveness of homeopathy is irrefutable. 13 years ago, the prestigious medical journal The Lancet ended the debate in an editorial entitled The end of homeopathy. elpais.com (Adapted) a) Imagine that you dissolve 1 g of glucose (C6H12O6) in water until it reaches 100 mL. What is its molarity? How many moles of glucose does 1 mL of this solution contain? b) Now add water to that 1 mL of solution until you have a final volume of 100 mL. What is the molarity of the glucose solution now? c) Use the results of the above calculations to prove that homeopathy is pseudoscience. Write a scientific report, applying the scientific method It is time to write the report for one of your scientific studies. Present it in the form of written text or as the script of an audiovisual production. You can start with a scientific problem from this unit or look for another one. Follow the scientific method, from observing to communicating the results. Remember that science has an important social responsibility. In addition to publishing in a journal, you should write an article to inform the general public. It is a good idea to include drawings or diagrams that help people to understand what you have studied and its consequences. While you were creating your report, you learnt the stages of the scientific method and applied them to the study of gases and solutions. W E L L D O N E ! CHALLENGE 33
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