November 14, 2012

CHE 120-15

Purpose:

The purpose of this experiment was to perform gravimetric analysis to quantitavely determine the amount of the sulfate ion in an unknown sample and compare individual results with an aggregated group sample by applying statistical methods.

Introduction:

Gravimetric analysis is a process by which the compound of interest (analyte) is precipitated as an insoluble compound and weighed [1]. In this experiment, the analtye BaSo4 was used due to its ability to form a solid, rather than an aqueous product. If volumetric determination of the analyte is provided, then the following steps can henceforth be used in gravimetric analysis. First, gravimetric determination requires a drying of the sample. If the sample is insoluble in water, is a small of amount of an acid added, in this case, Hall. To isolate and gather the formed precipitate, the solution will be heated slowly over a flame source. Once the precipitate has been isolated within the beaker, the crystalline product can then be filtered through filter paper and collected for weighing. Gravimetric analysis allows for quantifying the amount of sulfate within the sample.

The sulfate will be precipitated as a form of BaSO4 by the addition of BaCl2:

(Ba2+(aq) + SO42-(aq) BaSO4(aq)

Gravimetric determination of the percentage of sulfate mass and percentage will be determined from the application of the following formulas:

Mass of SO4 = __mass of SO4 x 100

Mass of the sample

% SO42- = mass of SO42- x 100

mass of sample

Propagation of error is a series of calculations performed using the uncertainties (errors) in a measurement. When the difference of 2 measurements are calculated to determine the mass of the unknown sample, the uncertainty associated with the result is determined by the formulas below:

* Mass of watch glass + sample = weight (g ) ± observed errorA (g) * Mass of watch glass alone = weight (g ) ± observed errorB (g) * Total mass of sample = difference of samples = ± observed error (g)

* Propagation of error = √(.A)2 + (.B)2 = propagation of error (g)

When 2 measurements are multiplied or divided, the uncertainty associated with the result is calculated by the same procedure, but the relative error for each measurement is used rather than absolute error.

Mass of SO42- = (propagation or error calculated)g of BaSO4

Mass of unknown sample = (difference of samples ± observed error)g

Percent of SO42- in unknown sample =__ (propagation or error calculated)g of BaSO4 __ x 100 (difference of samples ± observed error)g

Before grand average or standard deviation can be calculated, a Q-test must be performed to determine and if necessary eliminate any outliers. A Q-test can be calculated using the formula below, and the result compared to a chart listing confidence interval (see Table 7.1)[2]. If the derived value falls below the applicable table value, the value must be rejected. With the sample of 13 values obtained, the table denotes a value of .361.

Calculated = Spread = __(outlier – nearest value)_____

Range (highest value – lowest value)

Once this calculated has been done, a grand average of all samples can be calculated using the following formula:

X = _IA_

n

Standard deviation of the samples can now be calculated using the following formula:

s = √_Σ(xi – x)2_

n-1

It’s usually observed that the series of measures from group data collection tend to cluster around a central value. The central value can be asserted by using the above statistical analysis. The true mean and lower standard deviation will provide a more accurate measure of the percentage of SO4 in the sample, as the sample increases from individual to a group sample.

As per the experiment, gravimetric analysis...