81) According to the following thermochemical equation, what mass of HF (in g) must react in order to
produce 345 kJ of energy? Assume excess SiO2.
SiO2(s) + 4 HF(g) �� SiF4(g) + 2 H2O(l) DH°rxn = -184 kJ
81)
A) 173 g B) 37.5 g C) 150. g D) 107 g E) 42.7 g
wut
To find the mass of HF needed to produce 345 kJ of energy, we need to use stoichiometry and the given thermochemical equation.
The molar ratio between HF and the energy released is 4:345, meaning that for every 4 moles of HF, 345 kJ of energy is released.
First, calculate the number of moles of HF needed:
Molar ratio = 345 kJ / -184 kJ = -1.880
The negative sign is included because the reaction releases energy.
Now, use the molar mass of HF to convert moles of HF into grams:
Molar mass of HF = 1.01 g/mol + 19.00 g/mol = 20.01 g/mol
Mass of HF needed = moles of HF * molar mass of HF
= -1.880 mol * 20.01 g/mol
= -37.624 g
Since mass cannot be negative, we take the absolute value:
Mass of HF needed = 37.624 g
Therefore, the correct answer is B) 37.5 g.
To find the mass of HF that must react, we can use the concept of stoichiometry and the given information about the thermochemical equation.
First, let's calculate the amount of energy produced per mole of SiO2 reacted. The equation shows that for every mole of SiO2, -184 kJ of energy is released.
Next, we need to determine the number of moles of energy produced by 345 kJ. We can set up the following proportion:
-184 kJ / 1 mole = 345 kJ / x moles
Cross multiplying the above equation, we get:
-184 kJ * x moles = 345 kJ * 1 mole
Simplifying, we find:
x moles = (345 kJ * 1 mole) / (-184 kJ)
x moles = -345 kJ / 184 kJ
x moles ≈ -1.88 moles
Since stoichiometric coefficients in a balanced equation represent the mole ratio between the reactants and products, we know that for every 4 moles of HF, 1 mole of SiO2 is consumed.
Therefore, to produce -1.88 moles of SiO2 (based on the energy given), we need:
(-1.88 moles SiO2) / (1 mole SiO2) * (4 moles HF) = -7.52 moles HF
However, we assume excess SiO2, so all the HF will be used up. Therefore, we don't need to worry about the negative sign and can convert the moles of HF to grams using its molar mass.
The molar mass of HF is approximately 20 g/mol, so:
7.52 moles HF * (20 g/mol) = 150.4 g HF
Rounding to the correct number of significant figures, the closest answer choice is:
C) 150. g
184 kJ is produced by 4*HF = 4*20 = 80 g HF. Therefore, to produce 345, we will need
80 x (425 kJ/184 kJ) = ? g HF.