salt water mixtures living with the lab © 2011 David Hall the worlds oceans contain an average of about 3.5% salt by weight (35 grams of salt for every kilogram of seawater) Image credit: NASA/GSFC/JPL-Caltech

concentrations used for fishtank project 2 living with the lab 0.00% weight NaCl – we will actually use deionized water (DI water) 0.05% weight NaCl 0.10% weight NaCl 0.15% weight NaCl we will calibrate our salinity sensor using small NaCl concentrations

why not just use tap water? standard tap water contains dissolved ions such as sodium, calcium, iron, copper and bromide the electrical resistivity of tap water can vary widely (15kΩ-cm is typical) ions have been removed from DI water resulting in a much higher resistivity (18MΩ-cm is typical) 3 living with the lab Did you notice the strange units on resistivity (Ω-cm)? The electrical resistance R of a body (measured in Ω) is related to the electrical resistivity ρ (measured in Ω-cm) as A = cross sectional area L = length over which resistance is measured ρ = resistivity which is a material property

calculating percent weight A mixture contains 19 grams of water and 1 gram of NaCl. What is the weight percent of NaCl? 4 living with the lab

5 Class Problem If you add 9.5 grams of NaCl to 5 gallons of water, what weight percent of salt will the mixture contain?

6 living with the lab Example How much salt would you need to add to 2L of water to have a concentration of 3.5 wt% NaCl? unknown mass of 2L of water unknown 3.5%

recall the reactions at the electrodes 7 living with the lab e-e- 5V e-e- 10 k e-e- Cl - Cl Cl 2 Cl - e-e- e-e- e-e- OH - H2OH2O H2OH2O H H e-e- e-e- anode – oxidation (loss of electrons) cathode – reduction (gain of electrons) Cl - Na + Cl - Na + Cl - ion migration Na + Na + is a spectator ion oxidation occurs at the positively charged anode: reduction occurs at the negatively charged cathode:

useful information for reaction calculations 8 living with the lab

Class Problem Assume you have 5 gallons of water to which you add salt to create a mixture with 0.2 wt% NaCl. Determine: (a)the mass of the water (b)the mass of the salt (c)the number of moles of NaCl (d)the number of Cl - ions 9

living with the lab Example If a constant current of 0.1mA passes through the probes of the conductivity sensor, how many H 2 gas molecules would be formed over a 1 minute period? HINT: Use the definition of an amp and a Coulomb along with the chemical reaction at the cathode. two electrons pass through the circuit for each H 2 gas molecule formed Now, find the number of H 2 molecules over a 1 minute period. First, find the number of electrons that pass through the sensor per second. 10