Environmental Chemistry: Numerical Data - Assignment Example

Experiment 1 Name Student No: Unknown No: Introduction The objective of the experiment is to gain experience as far as the handling of numerical units is concerned. This is achieved by measuring some parameters of water composition. Environmental information is often expressed in form of numbers. Therefore, it is important to appreciate the meaning of numerical data. Numeracy constitutes an essential and important skill. It goes beyond calculation. When an understanding is reasonably developed on this topic, one will appreciate its application in significant figures, propagation of errors, types of errors, accuracy, and precision. In order to reveal the use and nature of these concepts, one has to use different methods to ensure the same environmental water parameter. In this experiment, TDS will be used to refer to Total amount of Dissolved Substances found in water sample. Common salts make up the dissolved solids for unpolluted waters. The Experiment Two methods are used to determine TDS in this experiment. The first method involves the evaporation of water from a sample. The residual mass is then measured. The second method involves the use of a conductivity meter. Experiment 1 This involves the gravimetric method. Therefore, during evaporation and drying steps of this method, the part of electrical conductivity was completed. Assumptions 1. The pure water conductivity is zero. The calibration line therefore passes through the origin and makes it a two points calibration. Conductivity is proportional to concentration of electrolyte, thus it is a linear function. The assumptions lead to a graph with a straight line through the origin where electrical conductivity is on the y-axis and KHP concentration is on X-axis. KHP is potassium hydrogen phthalate (KHP). METHODOLOGY The measured quantities as well as their uncertainty were recorded . Gravimetric method Assumption: evaporation removes solvent 1. A clean dry evaporating basin was taken from storage. It was weighed and its mass recorded to four decimal places (m1). The procedure was repeated and uncertainty in m1 was noted. Another balance was used to determine mass and uncertainty 2. 20mL of sample was pipette into the basin. The basin was placed on a steam bath. it was adjusted so that most part of the basin was immersed and was supported by the rim. The steam contact was meant to ensure slow evaporation of water without spilling any solids away. The basin was transferred to a 1050C oven after evaporation of water for complete drying. This took about 30 minutes. 3. Precision and accuracy of pipette While the basin was drying, a 20 mL of water was pipetted into a beaker and weighed. This was repeated for some times. The beaker was dried each time as the data was being recorded. The temperature of water was recorded 3 times. The standard deviation and mean volume were calculated. 4. The basin was transferred after 30 minutes to desiccators. It was cooled in a low humidity environment using silica gel. It was reweighed using balance 1. 5. The oven drying was repeated after 15 minutes until constant weight was recorded. The final mass was recorded in triplicate (m2) 6. The TDS was calculated and reported in mg/L 7. Electrical conductivity (EC) method Assumption: all dissolved materials contribute to conductivity since they are ionic. a 3000 mg/L KHP standard was prepared by dissolving 300 mg in water, which was 100 ml.Uncertainty for each piece of equipment was used to propagate concentration’s uncertainty. 8. A scale was set on a conductivity meter. This scale was 0-2000. Reverse osmosis was used to rinse the electrode. This was done until there was removal of all residues and until the conductivity reading came to less than 1. This point is zero reading. The procedure was repeated and the second value was obtained. The meter was calibrates by reading the value of EC of the standard solution 9. A linear calibration graph was constructed through a single calibration point of the standard solution. 10 the unknown was tested through the immersion of the electrode in the sample .the conductivity was read. The process was repeated to get the second value. Calibration graph was used to determine the unknown. Results Gravimetric Method Dry evaporating Basin (m1) Balance 1 Balance 2 m1 (g) 38.8976 38.72 Repeat m1(g) 38.8971 38.72 Uncertainty in mass (g) 0.00005 0.005 Average x (g) 38.8974 38.72 Standard deviation+-s(g) 0.00000034 0 Standard deviation for balance 1=(x-x-)2+(x-x-)2 =(38.8976-38.8971)2+(38.8971-38.8974)2=0.00052+-0.00032 =0.00000025+0.00000009=0.000000034 Standard deviation for balance 2=(x-x-)2+(x-x-)2 = (38.72-38.72)2 +(38.72-38.72)2=0 Precision and accuracy of pipette (V) 1 2 3 Average Standard deviation Mass of 20mL of pure water (g) (in triplicate) 19.9731 19.9978 19.9918 19.9876 0.0003145 Temperature (Oc) 28 28 28 28 0 Density of water (g/mL) based on average temperature 0.9962 0.9962 0.9962 0.9962 0 Calibrated Volume of 20Ml Pipette (V) 20 20 20 20 0 Standard deviation for mass of water=(x-x-)2+(x-x-)2 =(19.9731-19.9876)2 +(19.9978-19.9876)2 +(19.9918-19.9876)2 =-0.0145+0.0102+0.0014=0.0002103+0.000104+0.00000196=0.0003145 Drying of Residue 1 2 3 4 Record masses until constant mass is achieved (g) (15 minutes between weighing) 38.9564 38.9560 38.9550 38.9544 1 2 3 average Standard deviation Final Dry Mass m2 (g) (in triplicate) 38.9560 38.9550 38.9544 38.9551 0.00000131 Standard deviation=0.00092+0.00012+0.00072=0.00000081+0.00000001+0.00000049=0.00000131 Total Dissolved Solids (TDS) TDS (mg/L)=m2(mg)-m1(mg)/V(L) 38.8976 38.72 38.72-38.8976=0.1776/20=-0.00888g/L=8.88mg/L Note: use calibrated pipette volume Requirement: propagation of error for the final TDS calculation. The standard deviations for m1, m2 and V are used in this case. The formula is about addition and division. These are treated separately Propagation of error Standard deviation for m1=0.00000034 Standard deviation for m2=0.00000131 V=20ML TDS=-8.88 MG/l=+----------mg/L (propagated error) Conductivity method KHP Standard Measurement Absolute uncertainty Relative uncertainty Mass (mg) 0.3058 +- (mg) +-(%) Volume (L) 100 Concentration mg/L 0.003058 EC Meter Setup EC Meter Used 700 No.1 Meter range used 700No1 1 2 average Standard deviation EC Zero Reading 1.72 1.74 1.73 Creation of a two point calibration graph using the data for this standard.dtermination of the slope(rise/run) of the graph. The error of slope propagated from the uncertainty of the KHP concentration and the standard deviation for the KHP readings Unknown sample Meter Range Used The slope from the calibration graph is used to determine the TDS. The error can be estimated using both propagated error and standard deviation 1 2 Average Standard deviation EC Reading for Unknown 990 996 993 TDS (mg/L) Total Dissolved Solids (TDS) BY: TDS (mg/L) Gravimetric Analysis Conductivity Measurement Questions Precision and accuracy Q1.why is it important to use the same balance throughout an experiment? Q2 What did you determine the volume of 20ml pipette to be? How does your value compare to the manufacturers specification for accuracy of this pipette? Determine the most accurate and precise method for both your individual and class results. Provide evidence that support this. Propagated error Q 4 calculate the propagated error for your TDS determination for both the gravimetric and electrical conductivity methods Q. 5 Compare and estimate error determined using error propagation with the standard deviation for the electrical conductivity method of determining TDS. Which estimate of error is more likely to be a true indicator of the overall error in your result Significant figures Q. 6 How many significant figures apply for both the gravimetric and electrical conductivity Q. 7 Explain the implication of the number of significant figures found in Q4 in terms of accuracy for both the experimental techniques and calculations Types of errors Q 8. Describe two systematic and random errors present in the experimental procedure Q. 9 Name two mistakes most likely to be associated with data in the class results Read More