Question

FAN AND MEDAL Y"ALL!!

How many milliliters of magnesium metal, with a density of 1.74 g/mL, are needed to produce 102.3 grams of magnesium oxide in the single-replacement reaction below? Show all steps of your calculation as well as the final answer

Mg + H2O → MgO + H2

Answer 1

So, first step is to balance the equation?

is it balanced?

Next, you need to work "backwards", to find out how much you need, you first need to find out how much you produced in moles.

\(\sf moles=\dfrac{mass}{Molar~mass}\)

Answer 2

How do I find the amount of moles in this equation? I'm so bad a Chem :(

Answer 3

Use the mass given (102.3 grams of magnesium oxide) and the molar mass of the compound.

The molar mass can be found by adding the molar masses of the atoms in the formula (or easier by googling it).

Answer 4

So Google "molar mass of Mg + H2O → MgO + H2"

Answer 5

@aaronq

Answer 6

no, there's no such thing.

A molar mass is specific for a compound. In this problem, you need to find the moles of one of your products (MgO, magnesium oxide). So you need the molar mass of MgO only.

Answer 7

Okay hold on

Answer 8

Okay I got this 40.3044 g/mol

Answer 9

great. now find the moles of MgO (use the equation i posted above).

Answer 10

the mass/molar mass?

Answer 11

yep

Answer 12

I'm tryna figure out the mass, feel like an idiot

Answer 13

@aaronq

Answer 14

the mass is given in the question, 102.3 grams

Answer 15

102.3/ 40.3044 = 2.53818442

Answer 16

@aaronq

Answer 17

@abb0t help?

Answer 18

Looks as though someone is already helping. Bye.

Answer 19

great. Now you need to relate the moles of product to the moles of reactant (they're asking for Mg).

To do this, you use a ratio of the moles of each compound and their stoichiometric coefficients.

\(\sf \dfrac{moles~of~MgO}{MgO's ~coefficient}=\dfrac{moles~of~Mg}{Mg's ~coefficient}\)

plug in your values and find the moles of Mg.

(this step is actually sort of redundant but it applies to all problems, so do it anyway).

Answer 20

Are their coefficients 1?

Answer 21

@aaronq

Answer 22

yes. that's why it's redundant (for this specific exercise).

Now we know the moles of Mg.

We need to find the mass of Mg with same formula as before - but now with the molar mass of Mg.

Answer 23

24.3050 g/mol

Answer 24

okay, find the mass of Mg.

Answer 25

12?

Answer 26

@aaronq

Answer 27

nope. use: \(\sf moles=\dfrac{mass}{Molar~mass}\)

Answer 28

I'm trying to find mass tho?

Answer 29

SOOOOO lost

Answer 30

@aaronq

Answer 31

yeah, just rearrange the formula (it's just algebra).

\(\sf mass=Molar~mass*moles\)

Answer 32

How do I know how many moles are in the formula?

Answer 33

you found earlier (with the ratio), they're the same as the moles of MgO (because their coefficients are both 1)

Answer 34

2.53818442 * 24.3050 = 61.69

Answer 35

good stuff.

61.69 grams

now use the density to find the volume (mL)

the formula for density is: \(\sf density=\dfrac{mass}{Volume}\)

Answer 36

How do we know the volume?

Answer 37

you solve for it by using the mass and the density (this is given in the question)

Answer 38

So, 1.74 * 61.69 = 107.3406

Answer 39

nope. check your algebra

Answer 40

the density in the question is 1.74 tho...?

Answer 41

right, but the way you solved is wrong (algebraically).

the formula is: \(\rho =\dfrac{m}{V}\)

when you solve for V, it rearranges to \(V=\dfrac{m}{\rho}\)

Answer 42

okay so 61.69 / 1.74 = 35.45

Answer 43

@aaronq

Answer 44

yep

Answer 45

That is my answer?

Answer 46

yep. you should be writing the units always to know whats going on

Answer 47

Final answer to the question is 35.45 grams then?

Answer 48

@aaronq I was just missing the word grams?

Answer 49

nope, re-read the question. It's asking you for the mL (volume) of Mg.