This is because Lithium has isotopes that range from having 0 neutrons up to 13 but they all have different abundances. It has a mass number of 6.941 which in most periodic tables is rounded up to 7. Looking at another example Lithium has many isotopes. They are chemically the same, the only difference is Chlorine 35 contains 18 neutrons and Chlorine 37 contains 20 neutrons, both still have the same number of protons 17 and 17 electrons as you can see in the table. The key difference is the difference in mass number, this is caused by different number of protons. They have the same reactivity due to the number of electrons and also the same atomic number. As they have the same number of protons they will have the same atomic number and the same number of electrons. This is because in nature there there are two types of Chlorine atoms and they have different numbers of neutrons but the same number of protons. For example with Chlorine, the Atomic Mass is given as 35.5, this is because there are two forms of Chlorine as an element.
The empirical formula mass of a covalent compound may be compared to the compound’s molecular or molar mass to derive a molecular formula.Sometimes the Atomic Mass, unlike the Atomic number, is usually given as decimals. A compound’s percent composition provides the mass percentage of each element in the compound, and it is often experimentally determined and used to derive the compound’s empirical formula. The chemical identity of a substance is defined by the types and relative numbers of atoms composing its fundamental entities (molecules in the case of covalent compounds, ions in the case of ionic compounds). The molar mass is approximately 194 g/mol. Molar mass of molecular formula / Molar mass empirical formula = 90.3 g/mol / 46.0055 g/mol = 1.963 (to 3 SF) = 2Ĭaffeine contains hydrogen, carbon, nitrogen and oxygen.
Then, we need to use the molar mass values to determine the molecular formula:Įmpirical formula molar mass = 14.0067 g/mol + 2(15.9994 g/mol) = 46.0055 g/mol to 2 sig fig’s) is close enough to round to the whole number, or NO 2 The corresponding numbers of atoms (in moles) are: Consider a sample of compound determined to contain 1.71 g C and 0.287 g H. We then consider the moles of each element relative to each other, converting these numbers into a whole-number ratio that can be used to derive the empirical formula of the substance. To accomplish this, we can use molar masses to convert the mass of each element to a number of moles. Therefore, any experimentally derived data involving mass must be used to derive the corresponding numbers of atoms in the compound.
However, we must keep in mind that chemical formulas represent the relative numbers, not masses, of atoms in the substance. Thus, the empirical formula = C 12/2H 4/2Cl 4/2O 2/2= C 6H 2Cl 2Oĭetermine the empirical formula for the following compounds:Ī) C 3H 8N b) CCl 4 c) C 2H 5 Determination of Empirical FormulasĪs previously mentioned, the most common approach to determining a compound’s chemical formula is to first measure the masses of its constituent elements. Determine the empirical formula for dioxin (C 12H 4Cl 4O 2), a very powerful poison.