Question

How many moles of sand (SiO2) are in 30 g of sand?

Answer 1

Answer:

$\boxed {\boxed {\sf About \ 0.5 \ moles \ of \ SiO_2}}$

Explanation:

To convert from grams to moles, we must use the molar mass.

1. Molar Mass

Use the Periodic Table to find the masses of the individual elements (silicon and oxygen) in sand.

• Silicon (Si): 28.085 g/mol
• Oxygen (O): 15.999 g/mol

Examine the formula for sand: SiO₂. There is a subscript of 2 after oxygen, so there must be 2 oxygen atoms. Multiply oxygen's mass by 2 and add silicon's mass to find the molar mass of sand.

• SiO₂: 2(15.999 g/mol) + 28.085 g/mol= g/mol

2. Calculate Moles

Use the molar mass as ratio.

$\frac{60.083 \ g \ SiO_2}{1 \ mol \ SiO_2}$

Multiply by the given number of grams (30)

$30 \ g \ SiO_2 * \frac{60.083 \ g \ SiO_2}{1 \ mol \ SiO_2}$

Flip the fraction so the grams of sand will cancel.

$30 \ g \ SiO_2 *\frac{1 \ mol \ SiO_2}{60.083 \ g \ SiO_2}$

$30 *\frac{1 \ mol \ SiO_2}{60.083 }$

$\frac{30 \ mol \ SiO_2}{60.083 }$

$0.499309289 \ mol \ SiO_2$

3. Round

The original measurement of grams has 1 signfiicant figure. We must round our answer to 1 sig fig.

For the answer we found, that is the tenth place. The 9 in the hundredth tells us to round the 4 to a 5.

$\approx 0.5 \ mol \ SiO_2$

There are about 0.5 moles of SiO₂ in 30 grams.

Answer 2

Answer:

0.50 mol SiO₂

General Formulas and Concepts:

Math

Pre-Algebra

Order of Operations: BPEMDAS

1. Brackets
2. Parenthesis
3. Exponents
4. Multiplication
5. Division
6. Addition
7. Subtraction

• Left to Right

Chemistry

Atomic Structure

• Reading a Periodic Table
• Using Dimensional Analysis

Explanation:

Step 1: Define

30 g SiO₂ (sand)

Step 2: Identify Conversions

Molar Mass of Si - 28.09 g/mol

Molar Mass of O - 16.00 g/mol

Molar Mass of SiO₂ - 28.09 + 2(16.00) = 60.09 g/mol

Step 3: Convert

1. Set up:                                $\displaystyle 30 \ g \ SiO_2(\frac{1 \ mol \ SiO_2}{60.09 \ g \ SiO_2})$
2. Multiply/Divide:                  $\displaystyle 0.499251 \ mol \ SiO_2$

Step 4: Check

Follow sig figs and round. We are given 2 sig figs.

0.499251 mol SiO₂ ≈ 0.50 mol SiO₂