Mar. 3 Stoichiometry with grams to grams and moles to moles

We can use stoichiometry when doing calculations involving moles and grams.  We just need to know what to multiply and divide.

Ex. How man moles of O2 will be formed when 4.1 moles of H2O is decomposed?

4.1 mole H2 x 1 mole O2            =  2.1 moles O2
                                     2 moles of H2O

For these questions you always put the number of moles decomposed in the front of the equations and divide it by the number of moles of the element you are looking from the compound.  In this example we are looking for how many moles of  O2 was formed.  We know that there is 4.1 moles of water and that there are 2 moles of O2 in water so we divide by two. We get 2.1 moles as the answer.

We can do calculations involving particles -moles - mass

Ex. How many grams of Zinc will be needed to react with 5.20 moles of hydrochloric acid?

First we must write out the equation and we must balance it.

1 Zn + 2 HCL -> 1 ZnCl2 + 1 H2

Then we can do the calculation

5.20 moles HCI x 1 mole Zn     x  65.4 g Zn
                            2 mole HCL     1 mole Zn

= 170 g Zn

*when converting to grams to grams, we must go through this pattern: grams to moles to moles to grams

Ex.How many grams of NO is produced if 20 g of O2 is combined with excess ammonia?
4 NH3 + 5 O2 -> 6 H2O + 4 NO

20 g O2 x 1 mole O2  x  4 mol NO  x  30.01 g NO
                 32 g O2         5 mol O2      1 mol NO

= 15 g  
As you can see the fractions are placed in such an order that the tops and bottoms will cancel out and at the end cancel out to what you are looking for.  In the case, it cancels out in the end to NO.

Here is a video showing the calculations:

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