Question 98cef

Apr 28, 2015

Your unknown compound contained 0.250 moles of oxygen.

Here's one way of solving this problem. You know that your compound contained 0.250 moles of carbon and 0.500 moles of hydrogen.

You can use the molar masses of carbon and hydrogen to determine how many grams of each of these two elements were in the sample.

0.250cancel("moles C") * "12.0 g C"/(1cancel("moles C")) = "3.00 g C"

and

0.500cancel("moles H") * "1.00 g H"/(1cancel("mole H")) = "0.500 g H"

Since the compound only contains carbon, hydrogen, and oxygen, the difference between its total mass and the added masses of carbon and hydrogen will be the mass of oxygen.

${m}_{\text{compound" = m_"C" + m_"H" + m_"O}}$

m_"O" = m_"compound" - (m_"H"+ m_"C")#

${m}_{\text{O" = 7.50 - (3 + 0.500) = "4.00 g O}}$

This is equivalent to having

$4.00 \cancel{\text{g O") * "1 mole O"/(16.0cancel("g O")) = color(green)("0.250 moles O}}$