topic eleven Flashcards
(48 cards)
qualitative data
includes non-numerical data obtained from observations that are made during an experiment, not from actual measurements.
quantative data
includes numerical data (numbers), such as measurements taken in the laboratory during an experiment. Examples include mass, volume, temperature, voltage, pH, density and absorbance. Remember that when recording a measurement, there is an experimental error or degree of random uncertainty associated with the value.
SI units for electric current
amperes, A
Si units for temperature
kelvin, K
SI units frequency
hertz (reciprocal of seconds) Hz (s-1)
SI units pressure
newtons per square metre Pa (Nm-2)
examples of random errors
a breeze through an open door, which affects the mass measured on a mass balance
changes in the temperature of a room between morning and afternoon
errors of judgement when reading a scale on a measuring instrument.
random erros cannot be eliminated completely, but they can be reduced by
conducting repeat trials and taking an average of to the results
how to report measured quantities with a range of uncertainty
When taking a reading from analogue equipment (such as a measuring cylinder), you can estimate the value to within half of the smallest scale division. In the case of the measuring cylinder shown in Figure 1, the smallest scale division is 1 cm3, so half of the smallest division is 0.5 cm3.
systematic errors are caused by
a piece of apparatus has been wrongly calibrated
a scale is consistently misread by the experimenter
the apparatus is faulty.
examples of systematic errors include
not zeroing a mass balance before use (a non-zero error)
heat loss during an experiment to measure an enthalpy change
a leak in the apparatus used to measure a volume of gas.
parallax error
when a scale or measuring cylinder is read from the wrong position
systematic errors in determining enthalpay change
A significant proportion of the heat released by burning the alcohol is lost to the surroundings.
Heat is lost from the water and absorbed by the calorimeter itself.
random errors when determining enthalpy change
Random uncertainties when recording the masses of the alcohol and the water.
Random uncertainties when recording the temperature of the water during the experiment.
precision refers to
how close several experimental measurements or values are to each other
accuracy refers to
how close the readings or values are to the actual value
absolute uncertainity
The degree of uncertainty in a measurement is usually expressed in the same units as the measurement itself. This is the absolute uncertainty of the measurement, for example, 28.5 ± 0.1 °C
in order to calculate the total uncertainty
the absolute uncertainties can either be added together or converted to percentage uncertainties, which are then added together.
The equation used to calculate the percentage uncertainty is shown below:
Percentage uncertainty =
absolute uncertainty/measured value×100%
percentage error equation
Percentage error = (experimental value - theoretical value) / theoretical value × 100%
when plotting experimental data, the ind and dep variable goes on which axis
the independent variable is plotted on the x-axis and the dependent variable is plotted on the y-axis
When plotting a graph:
plot the independent variable on the x-axis and the dependent variable on the y-axis
label the axes and ensure your labels include the correct units
choose an appropriate scale for each axis
give your graph an appropriate title
draw a line of best fit (see below).
line of best fit
the line will not necessarily pass through all of the points; instead, you should try to ensure that approximately half the data points lie above the line, and half lie below. Outliers (points that do not seem to fit the overall trend) are not included in the data used to draw the line of best fit.
gradient at a particular point on a curve
interpolation, draw a tangent to the curve at that point and find the gradient of the tangent using the relationship, m=change in y / change in x
