Experimental Chemistry

CH2640_Problem_set_2_Answer_key.docx.docx-CH2640 Problem set II Answers Due

CH2640_Problem_set_1_Answer_key.docx.docx-CH2640 Problem set I (Answers) 1.

Answer Practice Quiz 3 Copper-Silver Phase Diagram 1. At 700 ֯ C, what is maximum wt% solubility of Cu in Ag? Show line on above graph. If I draw a horizontal line at 700 ֯C (left red line going from 100 wt% Ag), and it meets the curve line at roughly 94 wt% Ag, hence solubility of Cu in Ag is 6 wt% Cu. 2. At 800 ֯ C, what is the maximum at% solubility of Ag in Cu? Show line on the graph. Draw horizontal line at 800 ֯C from 100 wt% Cu (green line), roughly 9 wt% Ag is soluble in Cu and 5 at% Ag is soluble in Cu. 3. Is Cu-Ag system isomorphous? Explain your answer Cu-Ag is not isomorphous because at room temperature there are two phases present (α and β), whereas isomorphous system has 100% solubility of components into each other at room temperature or presence of only one phase at room temperature. 4. How much wt% of Cu is present in Cu-Ag system at eutectic point? At eutectic point, there is 71.9 wt% of Ag and 28.1 wt% of Cu. Please draw the line in the exam. 5. At eutectic point, how much of α-phase is present if I have 5 kg of Cu-Ag system? Using the level rule at eutectic point to calculate α-phase W α = 91.2 − 71.9 91.2 − 8.0 W α = 0.2319 There is 5 Kg in the system, hence there is 0.2319 * 5 = 1.16 kg of α phase 6. At what wt% of Cu, there is eutectoid reaction? Show line on graph Eutectoid reaction is point or temperature at which a solid phase transforms into two solid phase. In above phase diagram, δ solid phase transforms into two solid phases γ and ε as shown by red line. This transformation is at 75 wt% Zn and 25 wt% Cu. 7. At what temperature solidification start for 60 wt% Cu in Cu-Zn system? Show on the graph Solidification starts at 900 ֯ C as shown by black line in the graph. 8. At what temperature solidification ends for 60 wt% Cu in Cu-Zn system? Show on the graph Solidification ends roughly at 875 C as shown by black line in the graph. 9. Calculate wt% of Zn in α-phase at 600 ֯C and 40 wt% Zn in Cu-Zn system? 37 wt% Zn is present in α-phase at 600 C with 40 wt% Zn in the system as shown with blue line. 10. Write one thing that you would like to see change in this course?

CHEM | 2640 | Experimental Chemistry | assessment | Answer Practice Quiz 3 Copper-

1. Label all pernent components (i.e. cathode, anode, direcon of electron ﬂow, etc.) of this representaon of a voltaic cell given that: - The reacon Cu 2+ (aq) + 2e -  Cu (s) is occurring at one electrode, which is submerged in a sulfate soluon. - The overall E 0 for the cell is 0.575 V. Idenfy and label the second metal, also submerged in a sulfate soluon. a) Write the full, balanced redox reacon that is occurring between the two electrodes, including physical states of all species. b) Calculate the value of the reacon quoent when E = 0.615 V c) If the concentraon of CuSO 4 is increased, what eﬀect should this have on the cell potenal? (30 points) Labels: Cu(s), cathode; Ni(s), anode; voltmeter; salt bridge; electron ﬂow TO cathode, sulfate soluons a) Cu 2+ (aq) + 2e -  Cu(s) has E 0 red = 0.339 V E 0 = 0.575 V = E 0 red + E 0 ox = 0.339 V + E 0 ox E 0 ox = 0.236 V Nickel has E 0 red = -0.236 V but since Cu is being reduced, the other metal must be oxidized, so E 0 ox =+0.236 V for Ni Cu 2+ (aq) + Ni (s)  Cu(s) + Ni 2+ (aq) (2 electrons transferred) b) E = E 0 – 0.05916/n log Q 0.615 V = 0.575 V – 0.05916/2 log Q 0.04 = -0.02958 log Q log Q = -1.35 Q = 10 -1.35 = 0.044 Name___________________________________________________ CH2640 Exam I c) Since copper is the cathode and the reducon reacon is taking place, copper ions are being removed from the soluon and reduced as solid copper on the electrode. More copper ions in soluon will drive the equilibrium forward, favoring the reacon and increasing the number of mes that the reacon can occur. Mathemacally, since [Cu 2+ ] is in the denominator of the reacon quoent, increasing this number will make Q and log Q smaller. Since the cell potenal reacon subtracts log Q from the standard state potenal, this results in an overall larger (or less negave) value of E. 2. Given E probe = C + (0.05916/n)log[H + ], a) Explain why two buﬀer soluons are needed for the proper calibraon of a pH-meter electrode. b) If the meter recognizes pH 4 and 7 buﬀer soluons and a student uses contaminated pH 4.2 and 7.2 calibraon standards, explain which type of error this will introduce into subsequent soluon measurements. c) Calculate the percent error associated with the measurement of the pH of pure water (NOT from the GH306 DI tap…) using this improperly calibrated meter. (15 points) a) Each calibraon soluon used gives one set of (E probe , [H + ] values). Since there are essenally two constants (C and n) in the operang equaon for the meter, we need two points to solve for two unknowns. Also can be thought of as needing two points to ﬁnd the equaon of a line. b) A constant error that will cause all future measurements to be oﬀset by a parcular value (higher or lower) is a systemac error. c) pH of pure water = 7 (actual/correct value) Meter will read pH 0.2 units too low = 6.8 (experimental value) % error = (7-6.8)/7 x 100 = 2.86% 3. Describe the fundamental diﬀerence between determining water hardness through EDTA traon and with a conductance measurement. Which is more accurate at ﬁnding total hardness? (15 points) EDTA will only react with divalent caons (typically Mg 2+ and Ca 2+ in water samples). While these two ions are the most signiﬁcant players in terms of contribuons to water hardness, they are not the only ones. Conductance measurements will report a value based on contribuons of all ions in a water sample, not just divalent caons. This means that it includes other hardness-contribung ions, but also other ions that don’t necessarily contribute to water hardness. Arguments can be made for either method being more accurate at ﬁnding total hardness.

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