Stoichiometry Assignment

CHE 205

Tin (Sn) exists in Earth’s crust as SnO2, calculate the percent composition by mass of Sn and O in SnO2.
The formula for rust can be represented by Fe2O3. How many moles of Fe are present in 24.6 g of the compound?
What are the empirical formulas of the compounds with the following compositions? (a) 2.1 percent H, 65.3 percent O, 32.6 percent S, (b) 20.2 percent Al , 79.8 percent Cl.
Ammonia is a principle nitrogen fertilizer. It is prepared by a reaction between hydrogen and nitrogen.

                        3H2(g) + N2(g)                                   2NH3(g)

In a particular reaction, 6.0 moles of NH3were produced. How many moles of H2 and how many of N2 were reacted to produce this amount of NH3?


When the potassium cyanide (KCN) reacts with acids, a deadly poisonous gas, hydrogen cyanide (HCN) is given off. Here is the equation

      KCN (aq)    +     HCl (aq)                                KC l(aq) + HCN (g)

If a sample of 0.140 g of KCN is treated with an excess of HCl, calculate the amount of HCN formed, in grams.

(Hint – If one reactant is excess, the other reactant will be the limiting one)

Define limiting reagent and excess reagent. What is the significance of the limiting reagent in predicting the amount of the product obtained in a reaction? Can there be limiting reagent if only one reactant is present?
Potassium superoxide, KO2, is used in rebreathing masks to generate oxygen according to the reaction below. If the mask contains 3.50 moles KO2 and 2.98 mol water, how much moles of oxygen can be produced? What is the limiting reagent?

                   4KO2(s) + 2H2O(?) ? 4KOH(s) + 3O2(g)


Empirical Formula – a formula showing the smallest whole-number mole ratio.

Convert the grams of each element to moles.
To find the simplest ratio, divide each element’s mole value by the smallest mole value. This will ensure a something to 1 ratio.
If the mole ratios are NOT all whole numbers, you must multiply a whole number in order to obtain a whole number ratio.
It is helpful to recognize decimal forms of common fractions and multiply by the denominator.

Ex.  2.49 is close to 2 1/2 so multiply by 2 to get 5.

A compound is found to contain 36.48% Na, 25.41% S, and 38.11% O.  Find its empirical formula.
How many atoms are there in 5.10 moles of sulfur (S)?
How many grams of gold (Au) are there in 15.3 moles of Au?
Calculate the MOLAR MASS of the following

CH4 ——————————————————

            NO2 ——————————————————

           K2SO4   ———————————–

Choose the correct answer

What is the average mass, in grams, of one arsenic atom?
A) 48 ? 10–23 g    B)  33.0 g    C)  74.9 g    D)  1.24 ? 10–22 g    E)  8.04 ? 1021 g
How many molecules are there in 8.0 g of ozone, O3?
A) 3 molecules                                            D)      0 ? 1023 molecules
B) 6 ? 1024 molecules                             E)      6.0 ? 1023 molecules
C) 0 ? 1023 molecules
Ammonia reacts with diatomic oxygen to form nitric oxide and water vapor:

            4NH3 + 5O2?  4NO + 6H2O

            When 20.0 g NH3 and 50.0 g O2 are allowed to react, which is the limiting reagent?

And find the theoretical yield of Nitric Oxide ( NO)

Show calculation

Calculate the percent composition by mass of sodium in Na2CO3.

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