Empirical Formula from Percent Composition

Summary

This video tutorial explains how to determine the empirical formula of a compound from its percent composition. It covers the process of converting percent composition to mass, calculating moles, and finding the simplest whole-number ratio of atoms. With detailed examples and practice problems, the video also provides tips on rounding and adjusting for whole numbers when necessary. The steps ensure that viewers can confidently calculate empirical formulas for various compounds, making this an accessible and informative lesson for anyone learning chemistry.

Takeaways

• 😀 The empirical formula represents the smallest whole number ratio of atoms in a compound.
• 😀 For example, magnesium chloride (MgCl2) has a 1:2 ratio of magnesium to chloride ions.
• 😀 To determine the empirical formula from percent composition, assume a 100g sample.
• 😀 Convert the percentage of each element into grams (e.g., 17.6% hydrogen becomes 17.6 grams).
• 😀 Calculate the number of moles of each element using the molar mass.
• 😀 Keep extra significant figures in intermediate calculations to avoid rounding errors.
• 😀 Write the initial formula using the number of moles as subscripts for each element.
• 😀 Divide each subscript by the smallest number of moles to find the lowest whole number ratio.
• 😀 If a subscript is close to a whole number (e.g., 2.988 rounds to 3), round accordingly.
• 😀 If subscripts like 1.4 or 1.5 appear, multiply the formula by the smallest number to make all subscripts whole.
• 😀 In practice problems, always double-check your work and review steps to ensure accuracy.

Q & A

What is the empirical formula of a compound?

The empirical formula shows the smallest whole number ratio of atoms in a compound.

How is the empirical formula for magnesium chloride represented?

The empirical formula for magnesium chloride is MgCl2, indicating a 1:2 ratio of magnesium to chloride ions.

What is the first step in determining the empirical formula from percent composition?

The first step is to assume you have a 100-gram sample, which makes the percentages equal to the mass in grams for each element.

How do you calculate the moles of an element from its mass?

To calculate the moles of an element, divide the mass of the element by its molar mass (grams per mole).

Why is it important to keep extra significant figures during intermediate calculations?

It is important to keep extra significant figures to avoid rounding errors that may affect the final result.

What is the next step after calculating the moles of each element?

After calculating the moles, write the initial formula using the moles as subscripts for each element.

What do you do if the ratio of moles does not result in whole numbers?

If the ratio of moles does not result in whole numbers, divide by the smallest number of moles to get a ratio, and then round to the nearest whole number or multiply by the lowest factor to make them whole numbers.

What is the rounding rule for moles when you have decimals like 1.9 or 1.1?

For decimals like 1.9, you round up to 2, and for decimals like 1.1, you round down to 1.

How do you handle numbers like 1.3 when rounding to whole numbers?

For numbers like 1.3, you cannot round down to 1. Instead, multiply both elements by the lowest number that results in whole numbers.

What is the empirical formula for a compound with 57.4% carbon, 6.16% hydrogen, 9.52% nitrogen, and 27.18% oxygen?

After following the steps of calculating masses, moles, and dividing by the smallest number of moles, the empirical formula is C14H18N2O5.