C215 Final Exam-Questions with correct answers

percent yield - Answers- actual yield / theoretical yield percent error - Answers- (experimental - theoretical) / theoretical write the balanced equations for the neutralization reactions of NaOH and HCl CH3COOH and OH - Answers- HCl(aq) + NaOH(aq) = Nacl(aq) + H2O (l) CH3COOH(aq) + OH-(aq) = CH3COO-(aq) + H2O(l) is the acid or base the limiting reactant in each neutralization reaction - Answers- acid does it matter if your ΔT values are in degrees celsius or kelvin when using q=mcΔT equation and why - Answers- no, there is a constant difference between celsius and kelvin and we are only accounting for that difference if a reaction in a calorimeter increases the temperature of solution, is this an endo- or exothermic reaction, what is the sign of ΔH, and why - Answers- it would be exothermic because the heat produced is trapped and ΔH would be negative if both thermometers used in this lab were calibrated incorrectly and always read 1 degree higher than the real temperature, how does systematic error affect the calculated molar heat capacity of neutralization - Answers- it would not affect the molar heat capacity of neutralization because we only use the change in temperature, which would remain constant in this circumstance in figure 1, one data point was discarded and is not counted as final temperature, why is it discarded - Answers- it was outside the change in temperature region, therefore it could've been an error and could have affected the overall results if we included it predict if the following processes are exo- or endothermic and rationalize sign for ΔH. condensation of water steam. combustion of wood. nuclear fusion in the Sun. - Answers- a. exothermic, -ΔH b. exothermic, -ΔH c. exothermic, -ΔH two solutions were both at 20.0 °C initially, when they are mixed, the temperature of the solution became 18.0 °C, is the reaction between the two solutions endo- or exothermic and what is the sign of its ∆Hrxn - Answers- endothermic, +ΔH an unknown metal is either aluminum, iron or lead, if 150. g of this metal at 150.0 °C was placed in a calorimeter that contains 200. g of water at 25.0 °C and the final temperature was found to be 34.3 °C after thermal equilibrium was achieved, assuming heat was only transferred between water and metal, what is the identity of this metal specific heats, J/(g•°C): Fe (0.449) Pb (0.128) Al (0.903) H2O (4.184) - Answers- Fe H2A and BOH are acid and base and they react according to the following balanced equation: H2A(aq) + 2 BOH(aq) → B2A(aq) + 2 H2O(l) if 0.20 moles of BOH reacted with excess amount of H2A solution and 1500. J of heat was released by the reaction, what is the ∆Hrxn for the reaction as written above - Answers- -15,000 J NH4NO3, whose heat of solution is 25.7 kJ/mol, is one substance that can be used in cold pack, if the goal is to decrease the temperature from 25.0 °C to 5.0 °C, how many grams of NH4NO3 should we use for every 100.0 g of water in the cold pack, assume no heat was lost outside of cold pack, and the specific heat of the resulted solution was the same as water, or 4.184 J/(g•°C) - Answers- 35.2g hydrogen lamp has four emission lines with three different colors: red, aqua and violet, rank these three colors in increasing order of (1) wavelength; (2) frenquency; (3) energy - Answers- wavelength: red aqua violet frequency: violet aqua red energy: violet aqua red what is the purpose of calibrating the spectroscope - Answers- it will help us ensure our results are as accurate as possible and the spectroscope gives us an angle so we have to determine the relationship between the angle and wavelength so we can measure the spectrum of any source emission lines observed for hydrogen lamp are due to electrons moving from various orbitals with higher principle quantum numbers to a common orbital with a lower principle quantum number. what is the principle quantum number for this lower orbital that electron moves to - Answers- 2 in a hydrogen atom, which orbital has higher energy, 4s or 3d, how about in a neon atom - Answers- hydrogen: 4s neon: 3d arrange the following colors of light in order of increasing energy: violet, infrared, green, blue, microwave - Answers- microwave infrared green blue violet what is the energy, frequency and wavelength of photons invloved in a transition from n = 2 to n = 4 levels in a hydrogen atom, will photons be absorbed or emitted - Answers- energy: 4.09 E-19 J frequency: 6.17 E14 Hz wavelength: 4.90 E-7 m photons will be absorbed without any calculations, select all transition that will result in emission of photons and then rank these transition according to the wavelength of photons emitted (shortest to longest) a. n = 1 to n = 3 d. n = ∞ to n = 1000 b. n = 6 to n = 2 e. n = 10 to n = ∞ c. n = 10 to n = 100 f. n = 2 to n = 1 - Answers- d b f when Vernier readings (degree) were plotted on y-axis, and wavelengths of line-spectra (nm) for Hg-lamp were plotted on x-axis, the best-fit-line equation obtained from Excel was y = 0.204x + 131.5 predict where (on the Vernier reading) you may find the faint purple line in Hg-lamp, which was very challenging to find in this lab - Answers- 214.1 degrees which orbital (or orbitals) has the lowest energy for H atom, how about O atom? a. 2s b. 3p c. 3d d. 2p - Answers- H: 2s, 2p O: 2s according to Beer's Law, A=εbc, what should the slope and the intercept be in a calibration curve for a Beer's Law plot, keep in mind that Absorbance is plotted on y-axis, and Concentration is plotted on x-axis in the calibration curve - Answers- slope: εb y-intercept: 0 why is DI used for a blank when the standards are analyzed and a "prepared cereal blank" used when the measurements are recorded for the cereal sample - Answers- DI water is pure and it will not effect our experiment or have any of the substance that we want in it, the cereal blank will also not affect our wanted substance and DI water can also be used according to Beer-Lambert's Law (Eq. 3), what is the theoretical slope if absorbance is plotted on y-axis and concentration is plotted on x-axis and what is the unit of this slope in this lab when concentration has a unit of mg/mL, what is the unit of ε - Answers- the slope is absorbance/concentration, the units would be (au*mL)/mg, ε would have a unit of (au*mL)/(mg*cm) a student is using a Spec-20 to analyze the concentration of a solution, if the student measures the %T to be 52.6%, what is its absorbance (A=log(100/T)) - Answers- about 0.279 a 10-mL solution of CuCl2 gives an absorbance reading of 0.30 au at a specific wavelength, predict its absorbance at the same wavelength if 50-mL DI water is added to this solution - Answers- 0.05 permanganate ion (MnO4-) is purple in color. Assume its calibration curve is known and given below, if a KMnO4 solution has a % transmittance of 52.8%, what is its molarity concentration Absorbance = (251.7 L•mol-1) × (Fe Concentration) + 0.0972 - Answers- 7.16 E-4 M it is common to dilute stock solution to a specific concentration when it is used, how many mL of 6.0 M H2SO4 solution will be needed to prepare 10-L 0.1 M H2SO4 solution - Answers- 167 mL (M1V1=M2V2) a sample of 0.300 mg pure chromium was added to excess hydrochloric acid to form a 10.0 mL aqueous solution of a chromium (III) salt, which has a violet hue, exactly 1.00 mL of the resulting solution was analyzed using a spectrophotometer in a 1.00-cm cell at 575 nm, and the percent transmittance for the solution was 62.5%, what is the extinction coefficient, include proper unit for extinction coefficient - Answers- ε=6.8 (au*mL)/(mg*cm) are all the bonds in a PCl5 equivalent, according to the measured bond lengths, if not, which ones are longer - Answers- no, the bond length between the axial and central atom is longer than the length between the equatorial and central atom use detailed bond angles and repulsion to explain why some bonds are longer than others - Answers- repulsion pushes the terminal atoms as far away from each other as possible, because the bond angle between the axial and equatorial atoms is smaller, the distance between the axial and central atom has to be longer, so they can still be as far away as possible compare the bond angles in CH4, NH3, and H2O, how are the lone pairs affecting the bond angles and why - Answers- they all have the same bond angles but a different number of terminal atoms, CH4 does not have lone pairs, NH3 has a lone pair so its 3 terminal atoms are pushed as far away as possible resulting in a 109.47 degree angle, and H2O does the same thing with 2 lone pairs compare the energies of seesaw shape (putting lone pair in equatorial position) and umbrella shape: which has a lower energy(more negative) - Answers- seesaw