Counting Atoms online exercise for - Free Printable

Let's solve each compound step by step using coefficients (numbers in front of the formula) and subscripts (numbers written below and to the right of elements). We'll count the number of atoms for each element and then add them up for the total.

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1. K₂CO₃

- K: 2 (from subscript 2)
- C: 1 (no subscript, so it’s 1)
- O: 3 (subscript 3)

| Type of Atom | # of Atoms |
|--------------|------------|
| K | 2 |
| C | 1 |
| O | 3 |
| Total | 6 |

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2. Ba₃(PO₄)₂

This has a polyatomic ion (PO₄) with subscript 2, meaning everything inside the parentheses is multiplied by 2.

- Ba: 3 (subscript 3)
- P: 2 (since PO₄ has one P, and there are 2 PO₄ groups → 1 × 2 = 2)
- O: 8 (each PO₄ has 4 O atoms → 4 × 2 = 8)

| Type of Atom | # of Atoms |
|--------------|------------|
| Ba | 3 |
| P | 2 |
| O | 8 |
| Total | 13 |

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3. Na₂CrO₄

- Na: 2
- Cr: 1
- O: 4

| Type of Atom | # of Atoms |
|--------------|------------|
| Na | 2 |
| Cr | 1 |
| O | 4 |
| Total | 7 |

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4. 3 CaCl₂

There is a coefficient of 3 in front → multiply all atoms by 3.

- Ca: 1 × 3 = 3
- Cl: 2 × 3 = 6

| Type of Atom | # of Atoms |
|--------------|------------|
| Ca | 3 |
| Cl | 6 |
| Total | 9 |

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5. NH₄C₂H₃O₂

This is ammonium acetate. Let's break it down:

- N: 1
- H: 4 (from NH₄) + 3 (from C₂H₃) = 7
- C: 2
- O: 2

| Type of Atom | # of Atoms |
|--------------|------------|
| N | 1 |
| H | 7 |
| C | 2 |
| O | 2 |
| Total | 12 |

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6. 4 Al₂(CO₃)₃

Coefficient = 4 → multiply entire formula by 4.

First, find atoms in Al₂(CO₃)₃:
- Al: 2
- C: 3 (CO₃ has 1 C, and there are 3 CO₃ → 1×3 = 3)
- O: 9 (each CO₃ has 3 O → 3×3 = 9)

Now multiply by 4:
- Al: 2 × 4 = 8
- C: 3 × 4 = 12
- O: 9 × 4 = 36

| Type of Atom | # of Atoms |
|--------------|------------|
| Al | 8 |
| C | 12 |
| O | 36 |
| Total | 56 |

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7. Pb(NO₃)₂

- Pb: 1
- N: 2 (NO₃ has one N, and two NO₃ groups → 1×2 = 2)
- O: 6 (each NO₃ has 3 O → 3×2 = 6)

| Type of Atom | # of Atoms |
|--------------|------------|
| Pb | 1 |
| N | 2 |
| O | 6 |
| Total | 9 |

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8. 2 (NH₄)₂Cr₂O₇

Coefficient = 2 → multiply all atoms by 2.

First, analyze (NH₄)₂Cr₂O₇:
- N: 2 (from two NH₄ groups)
- H: 8 (each NH₄ has 4 H → 4×2 = 8)
- Cr: 2
- O: 7

Now multiply by 2:
- N: 2 × 2 = 4
- H: 8 × 2 = 16
- Cr: 2 × 2 = 4
- O: 7 × 2 = 14

| Type of Atom | # of Atoms |
|--------------|------------|
| N | 4 |
| H | 16 |
| Cr | 4 |
| O | 14 |
| Total | 38 |

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✔ Final Answers:

#### K₂CO₃
- K: 2
- C: 1
- O: 3
- Total: 6

#### Ba₃(PO₄)₂
- Ba: 3
- P: 2
- O: 8
- Total: 13

#### Na₂CrO₄
- Na: 2
- Cr: 1
- O: 4
- Total: 7

#### 3 CaCl₂
- Ca: 3
- Cl: 6
- Total: 9

#### NH₄C₂H₃O₂
- N: 1
- H: 7
- C: 2
- O: 2
- Total: 12

#### 4 Al₂(CO₃)₃
- Al: 8
- C: 12
- O: 36
- Total: 56

#### Pb(NO₃)₂
- Pb: 1
- N: 2
- O: 6
- Total: 9

#### 2 (NH₄)₂Cr₂O₇
- N: 4
- H: 16
- Cr: 4
- O: 14
- Total: 38

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