Finding the relative atomic mass of lithium

For this investigation I will determine the Relative atomic mass Ar by using two different methods. In the first method I will dissolve a piece of lithium of a known mass in water, I will then collect the hydrogen gas produced, which can be used to calculate the relative atomic mass of Lithium.


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In the second method I will titrate the resulting solution of lithium hydroxide with a known concentration of hydrochloric acid, this can also be used to calculate the relative atomic mass of lithium. By looking at the Stoichiometry ratio in the reaction I can see that the ratio of Li: H2 is , therefore the number of moles of Li will be twice as the number of moles of H2 in the reaction.

Atomic structure

Thus this caused inevitable error. Accuracy and reliability: In method 1 the procedure was very accurate; the equipment used is very accurate, however as mentioned before, the gas lost when replacing the bung caused a degree of inaccuracy. In method 2 not only were the results accurate but also precise.

ChemTeam: Calculate the average atomic weight from isotopic weights and abundances

Looking at the results table, the three accurate titrations fall within 0. Clearly method 1 and method 2 differ in accuracy and precision.

Find the Average Atomic Mass - Example: Magnesium

Method 1 is clearly less accurate and precise as the gas collected from the reaction of lithium and water was not. Show More. It exists to simplify the process of working out the mass of an atom or molecule.

Experiment to find the relative atomic mass of lithium

This would be difficult to deal with in practical situations, so scientists define the relative atomic mass of a carbon atom as 12 and work everything else out on that basis. Find the relative mass of any atom by adding the number of protons to the number of neutrons. Hydrogen has a relative atomic mass of 1, and carbon has a relative atomic mass of Isotopes of the same element have different numbers of neutrons, so you need to calculate for one specific isotope. Periodic tables show the relative atomic mass as the bottom number for an element, but this takes into account any isotopes.

Find relative molecular masses by adding up the contributions from each element.

Use the chemical formula to find how many of each atom is included, multiply their relative atomic masses by the number of atoms of each present, and then add them all up to find the result. Under this scheme, a neutral hydrogen atom has a mass of 1.


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You can think of this as counting each proton or neutron as 1 and ignoring the masses of electrons because they are so small in comparison. So the formula for relative atomic mass is simply:.