Introduction Studies have shown that, in order to conduct electricity, a substance must contain either free-moving electrons or free-moving ions; an ion is either a charged atom or a charged group of atoms (e.g., ammonium and hydrogencarbonate). In the solid state, an ionic compound consists of a lattice of oppositely charged ions which are not free to move because they are held together by strong electrostatic attractions. However, when an ionic compound is either melted or dissolved in water, these ions are free to move, and so the electrolyte formed does conduct an electric current.
Directly or indirectly, conductance is important in a variety of scientific contexts; e.g., nerve impulses, electroplating, electrical cells, and the extraction of metals by electrolytic reduction.
Quite reasonably, one might expect the conductance of an aqueous ionic compound to be dependent on several independent variables, including the concentration of dissolved compound ...
In this investigation, you are required to examine this hypothesis:  
As the concentration (M) of sodium chloride increases, within the 
range 0.00 - 1.00 mol dm-³, the conductances (C) of the aqueous
solutions increase in direct proportion; i.e., C = k × M.
1.  Using a measuring cylinder, measure out 100 cm³ of aqueous sodium
chloride (1.00 mol dm-³) into a clean beaker.
2.  Construct the circuit shown below, as follows: connect the series 
part first, ensuring that the positive output (+) of the d.c. source is 
connected to the positive input (+) of the ammeter ('red' to 'red'); 
then connect the voltmeter in parallel.
3.  Set the d.c. output source to a constant value (about 6 V ?).
4.  Close the switch, and immediately record the readings of both the 
ammeter and the voltmeter; if a meter reads below zero, then the
polarities are reversed.
5.  Open the switch, and record the following values: temperature of
the solution; depth of the immersed electrodes; and distance between
the anode and cathode.  These values should remain constant throughout
the following step.
6.  Ensuring that you clean and dry the electrodes between each set of 
readings, separately measure the current and voltage for the other
available concentrations and for appropriate duplicates.
Table of Results and Calculations
Constants: _________________________________________________________
 Concentration (M)
    / mol dm-³
 Current (I)
     / A
 Voltage (V)
     / V
 Conductance (C)
      / W
            [Conductance (C) = Current (I) ÷ Voltage (V)]
1.  Plot all the points of a well-labelled graph; i.e., complete with
units and with a precisely worded title.  Draw a best line through as 
points as is sensible; and then determine whether, broadly speaking, 
the graph is a straight line (go to 2), or a straight line and a curve 
(go to 3), or a curve (go to 4).
2.  If the graph is a straight line, determine its gradient 'k' (which 
has the units ohm-¹ mol-¹ dm³).  If this straight line passes through
the origin, then 'k' is the proportionality constant in the directly 
proportional relationship C = k × M (valid for the concentration range
0.00 - 1.00 mol dm-³).
3.  If the graph is a straight line and a curve, determine the gradient
of the straight line portion; this value, 'k', is the proportionality
constant in the linearly proportional relationship C = k × M + c (valid
for the concentration range __________ mol dm-³).
4.  If the graph is a curve, then these two variables are not in simple
proportion to each other; i.e., C ‡ k × M + c (over the concentration 
range __________ mol dm-³).
Notes (and Extension Work) 1. The SI unit for conductance is the siemen (S), but the equivalent unit of W-¹ is more commonly used (perhaps because it emphasizes that conductance is the inverse of resistance).
2. A graph shows direct proportionality between two variables, X and Y, if it is a straight line that passes through the origin (i.e., X = 0.0 and Y = 0.0); mathematically, this direct relationship is expressed as Y = k × X, where 'k' is the proportionality constant.
3. A graph shows linear proportionality between two variables, X and Y, if it is a straight line that does not pass through the origin; this linear relationship is expressed as Y = k × X + c, where 'k' is the proportionality constant, and 'c' is the intercept on the Y-axis.
4. A graph shows no simple proportionality between two variables, X and Y, if it is a curve; this absence of simple proportionality is expressed either as Y ‡ k × X, or, more generally, as Y ‡ k × X + c.
5. Complete this investigation, as follows. First, after considering the various sources of error, write a critical evaluation. Second, construct one or more precisely worded conclusions. And third, suggest a reasonable interpretation of the results.
6. Many substances are classified either as 'ohmic' conductors or, if their resistances change with voltage, as 'non-ohmic' conductors. Consider planning (and executing) an investigation which examines, over the voltage range 5 - 12 V, the ohmic characteristics of aqueous sodium chloride (1.00 mol dm-³).
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