Calculation of Molecular Formulas

Step 2 Determine the number of moles of each type of atom. We use the atomic masses to calculate moles. 

Step 3 Divide through by the smallest number of moles. 

The empirical formula for cisplatin is PtN2H6Cl2. Note that the number for hydrogen is slightly greater than 6 because of rounding-off effects. 

The most common form of nylon (Nylon-6) is 63.68% carbon, 12.38% nitrogen, 9.80% hydrogen, and 14.14% oxygen. Calculate the empirical formula for Nylon-6. 

Note from Example 8.14 that once the percentages are converted to masses, this example is the same as earlier examples in which the masses were given directly. 

OBJECTIVE To learn to calculate the molecular formula of a compound, given its empirical formula and molar mass. 

If we know the composition of a compound in terms of the masses (or mass percentages) of the elements present, we can calculate the empirical formula but not the molecular formula. For reasons that will become clear as we consider Example 8.15, to obtain the molecular formula we must know the molar mass. In this section we will consider compounds where both the percent composition and the molar mass are known. 

A white powder is analyzed and found to have an empirical formula of P2O5. The compound has a molar mass of 283.88 g. What is the compound s molecular formula  

We want to determine the molecular formula (PxOy) for a compound. That is, we want to solve for x and y. 

To obtain the molecular formula, we must compare the empirical formula mass to the molar mass. The empirical formula mass for P2O5 is the mass of 1 mole of P2O5 units. 

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A mass spectrometer that measures masses very accurately, usually to 1 part in 20,000. This high precision allows calculation of molecular formulas using the known atomic masses of the elements, (p. 545)... 

To calculate the properties of each gas in a mixture of gases To calculate molar masses from mass data along with pressure, volume, temperature data, and to use the molar masses thus calculated to enable calculation of molecular formulas from empirical formulas... 

Calculation of molecular formula from molecular weight http //sxl02a.niddk.nih.gov/software/massToFormula.html... 

J. Lederberg. Rapid calculation of molecular formulas from mass values./. Chem. Educ., 49 613,1972. 

Calculation of molecular mass from a molecular formula for several simple substances. Accurate masses are shown in parentheses. 

Explain why we cannot calculate a molecular formula for a compound of sulfur, sodium, and oxygen ... 

A compound consists of 85.7% C and 14.3% H. Its molecular weight is 42.0amu. (a) Calculate its empirical formula. (b) Calculate its empirical formula weight, (c) Calculate the number of empirical formula units in one molecule, (d) Calculate its molecular formula. 

Example A compound of nitrogen and hydrogen has an empirical formula NH2. The molar mass of the compound is 32 g mol-1. Calculate the molecular formula of the compound (relative atomic masses N = 14, H = 1). 

The concept of the molecular connectivity index (originally called branching index) was introduced by Randic [266]. The information used in the calculation of molecular connectivity indices is the number and type of atoms and bonds as well as the numbers of total and valence electrons [176,178,181,267,268]. These data are readily available for all compounds, synthetic or hypothetical, from their structural formulas. All molecular connectivity indices are calculated only for the non-hydrogen part of the molecule [269-271]. Each non-hydrogen atom is described by its atomic 6 value, which is equal to the number of adjacent nonhydrogen atoms. For example, the first-order Oy) molecular connectivity index is calculated from the atomic S values using Eq. (38) ... 

Usually the number of charges on an ion will not be known, but it can be calculated using a formula based on two different ions appearing in the spectrum. Actually, the molecular mass of a sample can be calculated automatically, or semiautomatically, by the processing software associated with the mass spectrometer. Experimentally, the automatic calculation of molecular mass is very helpful because a complex peptide or protein mixture will display an m/z spectrum with several overlapping series of multiply charged ions. 

To determine a moleculcir formula, you must know the gram formula mass of the compound as well as the empirical formula (or enough information to calculate it yourself from the percent composition see the preceding section for details). With these tools in hand, calculating the molecular formula involves three steps ... 

Use of the atom array to calculate a molecular formula and weight. 

A. Calculate the molecular formula of a compound whose empirical formula is CH2 and whose molecular mass is 126 amu. 

B. One allotropic form of sulfur has a formula S where x is a small integer. Its molecular mass is 256 amu. Calculate its molecular formula. 

Calculate the molecular formula of each of the substances from the following analyses ... 

White phosphorus is one form of elemental phosphorus. Its molar mass is 124 g/mol. Calculate its molecular formula. 

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