CHE 1302 Study Guide - Midterm Guide: Formula Unit, Sodium Chloride, Intermolecular Force

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sangriaskunk365

21 Sep 2019

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Here we are asked for a number of moles of ions and are given the volume and concentration of a solution. The molarity relationship is used as a conversion factor, and we must remember that. 2 ) for every formula unit. in this case we have three ions (two na+ and one co3. 0. 250 l 0. 391 mol na2co3 3 mol ions = 0. 293 mol ions. 1 l 1 mol na2co3: 0. 13, 1. 3 x 10-6 c) 13. Solution: here we are asked for the ppm (part/whole 1,000,000) of a 1. 3: the maximum allowable limit of copper in 1 l of water as set by the epa is. Firstly, we must have both numerator and denominator in the same units. 1. 3 mg is 1. 3 10-3 g, and we can covert from l of water to ml of water and then to grams of water (density of water is 1g/ml). This gives both the numerator and denominator in grams.

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Activity accounts for electrostatic interactions. For well-separated ions, electrostatic interactions are minimal; ions behave as at infinite dilution. As concentrations increase, distance between ions decreases and electrostatic interactions of force and energy increase. Consider the distance d between ions in solution. A spreadsheet is advisable. Find the equation for the distance between two ions in solution when the ions are present at a concentration c (mol/cm^3). In a spreadsheet, for log c from -12 to -2, calculate c and d. Increments of 0.5 in log c are appropriate. Units are c in mol/cm^3 and d in cm. Include calculation of concentration in molar units and separation distance in nm. What is the volume of one water molecule? What is the diameter of one water molecule in nm? Consider number of water molecules between ions as a rough estimate of the strength of electrostatic interactions between the ions. In the spreadsheet, how many water molecules are between two adjacent charges as a function of concentration? Plot log #waters versus log c(M). At what concentration is there about one water molecule between two ions? Comment on the shielding between two ions as concentration increases. In addition to shielding ions, water can be bound to ions. Once water is bound to ions, it is no longer free and therefore no longer part of the solvent. This increases the effective concentration of the ions because the solvent volume and mass has decreased. This is the Stokes Robinson model for activity, which is not explored in detail here. But, consider how bound water increases with c. (Part d is based in volumes, not distance between ions.) For a monocation, such as Na^+, the number of bound waters changes with ion size. For purposes of discussion, allow 5 bound waters per monocation. Anions such as Cl^- hold fewer waters; estimate this as 1. Then for every NaCl dissolved in solution, 6 waters are bound. To the spreadsheet, calculate the moles of each NaCl and water in cm^3. This calculates total moles of water per cm^3. Calculate the moles of free water at each concentration, which is total moles of water/cm^3 - 6 times moles NaCl/cm^3. Plot total moles of water, moles of free water, and moles of bound water per cc versus log c(M). At about what molarity of NaCl does the bound water begin to impact the free water? How does this correlate with the upper limit for Extended Debye Huckel (EDH)? Does EDH account for free and bound water? Density is 1.00 g/cm^3. Calculate volume of free water per cm^3. At what NaCl concentration does the model fail? How do you know? Calculate the effective concentration of NaCl as the moles of NaCl per cm^3 per volume of free water per cm^3. Calculate ratio of effective to real concentration of NaCl. What is c(M) where there is a ~2% change? a 20% change? a > 100% change? At about what concentration does free versus bound water become important? Calculate the mole fraction of NaCl (moles NaCl/(moles NaCl + total moles H_2O)) at each c. Report mole fraction in ppm. What mole fraction (ppm) corresponds to ~2%, a 20%, and > 100% change found in the just previous question? Fill in the blank or select an answer as appropriate. At ~ M, activity is not well modeled by extended Debye Huckel (EDH) as EDH does not account for free and bound waters. As concentration of ions increases, activity effects are more/less pronounced.

Im going to post the question being asked, an example of one I missed, and a refence that has some values you may need.

Thanks

Asking:

How many seconds are required to deposit 0.283 grams of copper metal from a solution that contains Cu²⁺ions, if a current of 0.941 A is applied?

Missed:

Chart:

CHE 1302 Study Guide - Midterm Guide: Formula Unit, Sodium Chloride, Intermolecular Force

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