Practical's.

• Rules for significant figures

  • Always count non-zero digits

  2. Never count zeros at the start of a number (leading zeros)

  3. Always count zeros that fall between two non-zero digits

  4. When a number with no decimal point ends in several zeros, these zeros may or may not be significant
• Examples of significant figures

  • 34.23 (4 s.f.)

  6000 (to 3 or 4 s.f.)

  2000.0 (5 s.f.)

  0.036 (2 s.f.)

  3.0212 (5 s.f.)
• Rules for rounding significant figures

  If the next number is 5 or more, round up

  2. If the next number is 4 or less, do not round up
• When combining measurements with different degrees of accuracy and precision, the accuracy of the final answer can be no greater than the least accurate measurement
• Significant figures


  The number of digits in a measurement that are known with certainty
• Never count the zeros at the start of a number when determining significant figures, even when there is a decimal point
• 3.0212 has five significant figures
• Rounding rules for significant figures

  1. If the next number is 5 or more, round up

  2. If the next number is 4 or less, do not round up
• For numbers in scientific form, to find the number of significant figures ignore the exponent (n number) and apply the usual rules
• Examples of numbers in standard form

  • 6.2091 x 10^28 has five significant figures
  • 1.3 x 10^2 has two significant figures
• The same number of significant figures must be kept when converting between ordinary and standard form
• Examples of converting to standard form

  • 0.0050 mol dm-3 = 5.0 x 10-3 mol dm-3 (2 s.f.)
  • 40.06 g = 4.006 x 10^1 g (4 s.f.)
  • 90.0 g = 9.00 x 10^1 g (3 s.f.)
  • 0.01070 kg = 1.070 x 10-2 kg (4 s.f.)
• Standard form makes it easier to identify significant figures
• Examples of rounding 260.99 to different numbers of significant figures

  • 4 s.f. is 261.0
  • 3 s.f. is 261
  • 2 s.f. is 260
  • 1 s.f. is 300
• In standard form, 2.350 x 10-3 has four significant figures, while 2.35 x 10-3 has three significant figures
• Water can also be squirted down the side of the flask to wash everything into the solution below. Although the solution is undoubtedly diluted, the amount, in moles, present in the pipetted solution will not have been changed and it is this that is being titrated.
• Sometimes solutions that are made up in volumetric flasks are not standard, for example an indigestion tablet can be reacted with a known excess of acid and the solution made up to 250 cm3 in a volumetric flask. The method is similar — the tablet is weighed in the beaker, a volume of acid added and the solution added to a volumetric flask, with rinsings, and made up to 250 cm3. Portions of this solution can then be titrated to determine, for example, the mass of calcium carbonate in the indigestion tablet.