EXPERIMENTAL: AN INVESTIGATION INTO THE EFFECT OF ACID CONCENTRATION ON
THE SPEED AT WHICH INDIGESTION TABLETS DISSOLVE IN HYDROCHLORIC ACID
Introduction Digestion is the name given to the series of physical and chemical processes by which complex insoluble foods are hydrolyzed to simple soluble substances that can be absorbed into the bloodstream. Dilute hydrochloric acid provides the acidic conditions necessary for stomach enzymes to continue the digestive process that starts in the mouth; however, an excess of stomach acid results in 'indigestion'. Tablets containing a base - such as aluminium hydroxide, calcium carbonate, magnesium hydroxide, or sodium hydrogencarbonate - are available which relieve indigestion by partially neutralizing the excess acid; e.g., 2HCl(aq) + CaCO3(s) 覧覧覧覧覧 CaCl2(aq) + H2O(l) + CO2(g)
In this investigation, you are required to examine two related
hypotheses.
Hypothesis 1: The time (T) taken for __________ indigestion tablets  
to dissolve in aqueous hydrochloric acid is in linear proportion to 
the concentration (C) of the acid; i.e., T = k ラ C + c.
Hypothesis 2: The speed (S) of dissolving of __________ indigestion
tablets in aqueous hydrochloric acid is in linear proportion to the
concentration (C) of the acid; i.e., S = k ラ C + c.
Method
1.  Using a reference thermometer, measure room temperature.
2.  Using a measuring cylinder, place 25 cmウ of aqueous hydrochloric
acid (2.00 mol dm-ウ) into a scrupulously clean beaker.
3.  Simultaneously, add one tablet to the acid and start the clock.
4.  Stop the clock when the tablet has completely dissolved.
5.  Repeat steps 2 to 4, but each time use a different concentration of
acid; the water should be at the same temperature as the stock solution
of acid, and the volume of water should be measured using a separate 
measuring cylinder.
6.  Repeat steps 2 to 5 for appropriate duplicate experiments.
7.  Using the reference thermometer, re-measure room temperature.
Table of Results and Calculations
Constants: room temperature (_____); initial concentration of acid 
(2.00 mol dm-ウ); volume of solution (25 cmウ); brand of indigestion 
tablets (__________); surface area of each tablet (_____); mass of 
each tablet (_____); absence of catalysts.
Volume of
 HCl(aq)
  / cmウ
Volume of
   H2O
  / cmウ
Final Conc. of
  HCl(aq) (C)
   / mol dm-ウ
Time for tablet
to dissolve (T)
     / s
Speed of tablet
dissolving (S)
     / ms-ケ
    25
    0
     2.00
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
     5
   20
     0.40
 
 
 
 
 
 
 
[Speed (S), the inverse of time (i.e., T-ケ), is calculated using
either of these equally correct equations: speed (in s-ケ) = 1  time 
(in s) or speed (in ms-ケ) = 1000  time (in s).]
Graphs
1.  Plot all data points of a first graph, with the dependent variable
of time (T) on the vertical axis.  Draw a best line through as many
points as is sensible; here, you should obtain a curve.
2.  The tabulated data and this first graph should provide the evidence
for this conclusion: "In the absence of catalysts, the time (T) taken
for one whole __________ tablet to dissolve at room temperature (_____)
decreased as the concentration (C) of aqueous hydrochloric acid
increased (within the range ____________ mol dm-ウ). The curve indicates
that these two variables are not in linear proportion to each other; 
i.e., T  k ラ C + c."
3.  Plot all data points of a second graph, with the dependent variable
of speed (S) on the vertical axis. As here you should obtain a straight
line, determine its gradient (units are ms-ケ mol-ケ dmウ); this value,
'k', is the proportionality constant in the linearly proportional
relationship S = k ラ C + c.
Sources of Error and Calculation Broadly speaking, experimental work can be considered to include three types of error: systematic (e.g., those arising from inaccuracies in each instrument used); random or observational (e.g., those arising from inaccuracies in reading each instrument); and design (i.e., those introduced by assumptions inherent in the experimental method). 1. Name the instruments which introduce systematic errors into these experiments. __________________________________________________________ 2. Estimate reasonable values for the probable errors in your reading of each of these instruments. _________________________________________ 3. Suggest one or more possible errors inherent in the experimental method. _______________________________________________________________ _______________________________________________________________________ 4. Use the second graph to form a second, precisely worded conclusion from your investigation. ______________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________ _______________________________________________________________________
Notes 1. To gain an outline of the range of results, it is often useful to to measure first the dependent variable for the largest and smallest values of the independent variable; in this investigation, for example, these are C = 2.00 mol dm-ウ and 0.40 mol dm-ウ, respectively.
2. To be able to determine confidently a relationship between two variables, it is necessary to measure the dependent variable for at least five values of the independent variable; in this investigation, for example, a reasonable choice would be C = 2.00, 1.60, 1.20, 0.80, and 0.40 mol dm-ウ. [For absolute rigour, a control experiment could be be executed; that is, C = 0.00 mol dm-ウ.]
3. To minimize experimental errors, as well as to gain some idea of the reproducibility of both method and technique, it is necessary to execute duplicate (or confirmatory) experiments. There are two methods of choosing 'duplicates': either arbitrarily selecting the middle value of the independent variable (in this investigation, for example, this would be C = 1.20 mol dm-ウ); or, with scientific rigour, examining the raw data for a value of the dependent variable which does not follow the general pattern (the detection of this value, known as an anomalous result, occasionally requires drawing a sketch graph).
Dr. R. Peters Next Contents' List