Aim: How to determine Empirical and Molecular Formula DO NOW: Here is data from an experiment: 1.Mass of empty crucible + cover = g 2.Mass of crucible + cover + hydrated crystal (before heating) = g 3.Mass of crucible + cover + anhydrous salt (after heating) = g What is the % by mass of the water in the hydrated crystal?

Empirical Formula vs. Molecular Formula Empirical Formula tells us what elements are present in the compound in the simplest whole number ratio of elements. Molecular formula tells which elements are present in the compound and the actual number of each. How to determine empirical and molecular formula

Empirical Formula from Masses of Elements A sample weighing g of a compound contains g N and g O. What is the empirical formula of the compound? This compound has the formula N x O y. We need to convert the mass of N and O to moles of N and O. Then divide each by the smallest mole to get a whole number ratio.

Solution

How to determine empirical and molecular formula

Empirical Formula from Percent Composition An analysis of sodium dichromate gives the following percentages: 17.5% Na, 39.7% Cr, and 42.8% O. What is the empirical formula of this compound? This compound has the formula Na x Cr y O z. Assume that you have an g sample of the compound. Then the mass of each element in the sample equals the numerical value of the percentage.

Solution How to determine empirical and molecular formula

Solution Now you divide all mole numbers by the smallest one. For Na: (0.761 mol)/(0.761 mol) = 1.00 For Cr: (0.763 mol)/(0.761 mol) = 1.00 For O: (2.68 mol)/(0.761 mol) = 3.52 The subscripts are not all whole numbers. To be made into whole numbers multiply each one by 2 to get an empirical formula of Na 2 Cr 2 O 7.

Molecular Formula from Empirical Formula The molecular formula of a compound is a multiple of its empirical formula. The molecular formula may be determined by dividing the molar mass of the compound by the empirical molar mass. When you divide you find which multiple it is of the empirical formula. How to determine empirical and molecular formula

Determining Molecular Formula

Step 2: Divide each mole number by the smallest mole number and round to the nearest integer For C: = 6.67/6.67 = 1 For H: = 20.0 / 6.67 = 3 The empirical formula is CH 3 How to determine empirical and molecular formula

Step 3: Determine molar mass of empirical formula CH 3 = C + 3H = (12.0 g/mol) + (3 x 1.0 g/mol) = 15.0 g/mol How to determine empirical and molecular formula

Step 4: Divide molecular molar mass of compound (75. 0 g/mol) by empirical molar mass. How to determine empirical and molecular formula

Practice What is the molecular formula of a compound that has an empirical formula of NO 2 and molecular mass of 92.0 g? How to determine empirical and molecular formula