Calorimetry Lab Report

I. Objective: The objective of this experiment is to determine the mass percent of iron in an iron compound using a spectrophotometer. From there, determine which iron compounds are in the stock room bottles based off of the experimental mass percent results.
II. Introduction: The objective is to determine the mass percent of iron in an iron compound using a spectrophotometer. From there, determine which iron compounds are in the stock room bottles based off of the experimental mass percent results. The objective is going to be met by first using absorption spectroscopy. This will be done by making 6 dilutions of a known compound of FeNH4(SO4)2 . Absorption spectroscopy involves placing the 6 diluted solutions into the spectrophotometer. This will measure the light absorption of the individual dilutions. The absorption values will be the y values on the Beers Law Plot. Beers Law shows that there is a relationship between absorption and concentration so the x value on the Beers Law Plot will be the concentration of the 6 diluted iron solutions. To calculate the concentration, the equation is M1V1=M2V2 solving for the final molarity. Then the
concentrations will be plotted with the corresponding absorbance values. A point on the trend line will be chosen to model the volume of the iron compound and water for the unknowns. Four unknown iron solutions will be prepared from this model and each individually run through the spectrometer to determine an absorbance value. To find the molarity of the compound diluted, the absorbance value of the compound will be plugged in for the y value of the equation and x will be calculated using the epsilon value. This x value calculated will be the concentration of iron in diluted. The equation M1V1=M2V2 will be used to solve for M1 to find the concentration of the iron in undiluted. This will generate a concentration that can then be multiplied by the molar mass of iron to be converted into grams. The amount in grams will then be divided by the density labeled on the stockroom bottle and multiplied by 100 to give a mass percent.

III. Data and Results:
1. Reagents Used and Concentrations from bottles.

Reagents | Concentration | FeNH4(SO4)2∙12H2O | 0.005002 M | HNO3 | 6 M | KSCN | 0.1004 M | Unknown 1 | 1.1618 g unk/L | Unknown 2 | 2.5262 g unk/L | Unknown 3 | 1.1484 g unk/L | Unknown 4 | 1.4590 g unk/L |

2. Known Solutions Trial Number: | 1 | 2 | 3 | 4 | 5 | 6 | Volume FeNH4(SO4)2∙12H2O mL | 2.00 | 1.80 | 1.60 | 1.40 | 1.20 | 1.00 | Volume HNO3 mL | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | 1.00 | Volume KSCN mL | 2.00 | 2.00 | 2.00 | 2.00 | 2.00 | 2.00 | Volume water | 15.00 | 15.20 | 15.40 | 15.60 | 15.80 | 16.00 | Concentration Fe3+ (M) | 0.000500 | 0.000450 | 0.000400 | 0.000350 | 0.000300 | 0.000250 | Absorbance Value | 1.509 | 1.402 | 1.218 | 1.995 | 0.9255 | 0.7148 | Wavelength of Amax (nm) | 462 | M Absorbtivity Coefficient | 3177.1 | Correlation coefficient | 0.9938 |

Attach or insert Beer’s law plot.

3. Unknown Solutions
Unknown Letter ______A__________ Trial Number: | 1 | 2 | Volume Unknown 1 | 1.60 | | Volume HNO3 | 1.00 | | Volume KSCN | 2.00 | | Volume water | 15.40 | | Absorbance Value | 1.130 | | [Fe3+] in diluted | 0.000372 | | [Fe3+] in undiluted | 0.00466 | | Mass Fe (per Liter) in undiluted | 0.260 | | Percent Fe | 22.4% | | Average % Fe | 22.4% | Identity of unknown | Fe2(SO4)3 * 5H2O |

Unknown Letter ______B_______ Trial Number: | 1 | 2 | Volume Unknown 1 | 1.60 | | Volume HNO3 | 1.00 | | Volume KSCN | 2.00 | | Volume water | 15.40 | | Absorbance Value | 1.344 | | [Fe3+] in diluted | 0.000406 | | [Fe3+] in undiluted | 0.00508 | | Mass Fe (per Liter) in undiluted | 0.284 | | Percent Fe | 11.2% | | Average % Fe | 11.2% | Identity of unknown | Fe(NH4)(SO4)2 * 12H2O |

Unknown Letter _____C______ Trial Number: | 1 | 2 | Volume Unknown 1 | 1.60 | | Volume HNO3 | 1.00 | | Volume KSCN | 2.00 | | Volume water | 15.40 | | Absorbance Value | 1.030 | | [Fe3+] in diluted | 0.000341 | | [Fe3+] in undiluted | 0.00426 | | Mass Fe (per Liter) in undiluted | 0.238 | | Percent Fe | 20.7% | | Average % Fe | 20.7% | Identity of unknown | FeCl3 * 6H2O | | |

Unknown Letter ____D_______ Trial Number: | 1 | 2 | Volume Unknown 1 mL | 1.60 | | Volume HNO3 mL | 1.00 | | Volume KSCN mL | 2.00 | | Volume water mL | 15.40 | | Absorbance Value | 0.8869 | | [Fe3+] in diluted (M) | 0.000296 | | [Fe3+] in undiluted (M) | 0.00370 | | Mass Fe (per Liter) in undiluted (g) | 0.207 | | Percent Fe | 14.2% | | Average % Fe | 14.2% | Identity of unknown | Fe(NO3)3 * 9H2O |

IV. Discussion of Results: Generating the Beer’s Law Plot of the known iron compound is necessary because when calculating the absorptivity of the unknown iron compounds, the compounds must fall in the range of the Beer’s Law Plot of the known iron compound. The Beer’s Law Plot generated shows a steady increase in absorbance as molarity increases. This is a fairly successful Beers Law Plot because all of the points were linear enough to easily make a trend line with all of the points in close range. The generated epsilon value was 3177.1. This is the molar absorptivity coefficient related to the wavelength. This value is a part of the Beer’s Law equation and was used to calculate the molarity of iron in diluted. Unknown solution A was Fe2(SO4)3 * 5H2O. Unknown solution B was Fe(NH4)(SO4)2 * 12H2O. Unknown solution C was FeCl3 * 6H2O. Unknown solution D was Fe(NO3)3 * 9H2O. As previously stated, the mass percent of iron was calculated for each unknown. In order to identify which compound belonged to which unknown bottle, the theoretically mass percent of iron in each compound was calculated. This was done by taking the amount, in grams, of iron in each compound and dividing it by the total molar mass of the compound and then multiplying it by 100. This allowed for a comparison of the theoretical and experimental results of the mass percent of iron in the compounds and allowed for each unknown to be identified. In regards to precision, each unknown was fairly precise among 7 groups. For each compound there was always one out of the seven groups whose value deviated largely from the others. For unknowns A and C the standard deviation was +- 2 which shows that the trials did not have great precision but the value is largely due to the one or two deviating results of groups. Unknown B had very great precision, which was reflected in a standard deviation of +- 0.7. This can also be noted in the trials that all the results were extremely close to one another. Unknown D had poorer precision with a standard deviation of +-3 which can largely be attributed to a large deviation of two results. This experiment could have definitely been more precise, however, if one trial was disregarded in each unknown, the precision definitely would increase and have more positive results. The poor precision could be attributed to human and experimental errors, which can occur quite frequently with this particular experiment. One human error that definitely occurred was the incorrect measurement and delivery of liquid using the pipets. Instead of stopping at the line marked zero when delivering the liquid, the two students using the pipets delivered the liquid that was located in the part of the pipet that is not calibrated. Therefore more than one or two milliliters of the designated liquids were delivered. Many other students may have been doing this in other groups as well which could account for the varying mass percent measurements. Another human error that may have occurred was the incorrect reading of instruments. Many people often misread pipets and burets and often do not correctly measure the meniscus, which could make a difference in the results obtained. Experimental errors may also have occurred such as instruments being calibrated incorrectly. This could cause the measurements to be different than what the individual intended to obtain without any fault of the individual. If an instrument is calibrated incorrectly, one would never know and it could just lead to poorer results. Another experimental error that could occur is involved with the spectrophotometer. If the spectrophotometer isn’t working properly it can be difficult to tell. It could be calculating incorrect absorbance if there was something wrong with the light absorptivity. No one would know so it would not be the fault of the experimenter but it could change the results of the experiment. For unknown A, the theoretical mass percent is 22.8% where as this particular experiment achieved 22.4% which had a good percent error of -1.7%. For unknown B, the theoretical mass percent is 11.6% where as this particular experiment achieved 11.2%. This also had an okay percent error of -3.4%. For unknown C, the theoretical mass percent is 20.7% and 20.7% was obtained for this experiment. So this achieved exact results. For unknown D, the theoretical mass percent is 13.8% and the obtained mass percent was 14.2%, which had a good percent error of 2.9%. So the accuracy for this experiment was good but it could have been better for unknown B and D. For all of the groups, the accuracy varied. Some groups did very well while others were far from the theoretical value. So there was a lot of variance between the seven groups in regards to accuracy. But, Group 4 in particular had fairly accurate measurements.

V.
Conclusion: Unknown solution A was Fe2(SO4)3 * 5H2O. Unknown solution B was Fe(NH4)(SO4)2 * 12H2O. Unknown solution C was FeCl3 * 6H2O. Unknown solution D was Fe(NO3)3 * 9H2O. The professor should have confidence that these are the correct identifications for the bottles because although the groups as a whole did not have the best precision and accuracy, it was mainly due to one or two groups having poor results. On a group to group basis though, there was very good accuracy. The results of each compound clearly identified the unknown with one of the iron compounds. There was no guessing which unknown was which compound because each experimental mass percent matched up well with the theoretical mass percent of the iron compounds. So a lot of confidence is behind the accuracy of the identification of the unknown
compounds.

VI. References:
Bauer, Richard, James Birk, Doug Sawyer. General Chemistry Lab Experiments: Northern Kentucky University. Cengage Learning. Mason, Ohio, 2011.

Kotz, John, Paul Treichel, John Townsend. Chemistry and Chemical Reactivity Eighth Edition. Brooks/Cole Cengage Learning. Belmont, California, 2012.