Chemistry Practice Problems Standard Temperature and Pressure Practice Problems Solution: An herbicide is found to contain only C, H, N, and...

Solution: An herbicide is found to contain only C, H, N, and Cl. The complete combustion of a 100.0-mg sample of the herbicide in excess oxygen produces 83.16 mL of CO2 and 73.30 mL of H2O vapor at STP. A separate analysis shows that the sample also contains 16.44 mg of Cl.Calculate its empirical formula.

Problem

An herbicide is found to contain only C, H, N, and Cl. The complete combustion of a 100.0-mg sample of the herbicide in excess oxygen produces 83.16 mL of CO2 and 73.30 mL of H2O vapor at STP. A separate analysis shows that the sample also contains 16.44 mg of Cl.

Calculate its empirical formula.

Solution

We are asked to determine the empirical formula of the herbicide containing C, H, N, and Cl. 

The complete combustion of a 100.0-mg sample of the herbicide in excess oxygen produces 83.16 mL of CO2 and 73.30 mL of H2vapor at STP. A separate analysis shows that the sample also contains 16.44 mg of Cl.

Recall:

Standard temperature and pressure (STP) is defined as 0°Celsius, and 1.00 atmospheres, respectively. Avogadro determined that the volume of any gas measured at STP is 22.4 L. This value is used in many gas conversions from liters to moles, or vice-versa, as long as the STP conditions are met

The empirical formula is a chemical formula showing the simplest (or lowest) whole number ratio of elements in a compound.

To solve this problem, we shall follow these steps:

Step 1: Convert the given quantities from mg to g and from mL to L.

Step 2: Convert volumes of products to moles of products (mol/22.4 L).

Step 3: Perform a mole-to-mole comparison (1 mol C:1 mol CO2; 2 mol H: 1 mol H2O).

Step 4: Determine the mass of C and H using their respective molar mass.

Step 5: Determine the total mass of C, H, and Cl. From here, determine the mass of N.

Step 6: Determine the moles of N and Cl using their respective molar mass. 

Step 7: Determine the empirical formula by dividing the moles with the smallest number of moles.

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