Chemistry with stoichiometry and empirical formula

Question

A compound that does not contain oxygen is combusted to give: a) 33.75 g of water b) Carbon dioxide collected under water in a 37.5 L vessel at 20 degrees Celsius and an atmospheric pressure of 752.2 torr. c) Sulfur dioxide which is collected by dissolution in water. This aqueous solution is titrated (reacted) with a 1.85M solution of sodium hydroxide. To reach the equivalency point, 404.5 mL of NaOH solution were consumed.

 

The vapor pressure of P(H2O)= 17.5 mmHg @20.0 Degrees Celsius.

The equivalency point is when enough base was added to consume all the acid present.

1. Determine the molecular formula of the compound if the molar mas of the compound is 179 g/mol as determined by the mass spectrometry.

2. Write a balanced equation for the combustion reaction and the titration.

3. Calculate the volume (L) of oxygen at STP needed for the combustion of 120 g of this compound. 

 

If you can even just answer a part of this, it would be greatly appreciated! 

 

Answer 1

The way to solve this problem is to figure out how many moles of water, carbon dioxide, and sulfur dioxide are formed from the reaction.  Then determine the empirical formula of the compound by finding the lowest whole number ratio of the components. ( C_xH_yS_z )  Then determine the molecular formula using the given molar mass.

 

The moles of water can be calculated by dividing the given mass by the gram formula mass of water.

 

The moles of carbon dioxide can be found using the ideal gas law equation PV=nRT.  The pressure must be corrected for the vapor pressure of water (Dalton's Law of partial pressure). The Kelvin temperature is obtained by adding 273.15 to the Celsius temperature.

 

The moles of sulfur dioxide can be obtained from the titration results.  Sulfur dioxide reacts with water to form sulfurous acid (H₂SO₃).  2 moles of sodium hydroxide are required to completely react with each mole of sulfurous acid.

 

Once you have the molecular formula you can write the balanced equation and determine the moles of oxygen required in the reaction.  The moles can be converted to liters using the conversion factor that 1 mole of an ideal gas occupies 22.4 liters at STP