When an ionic compound dissolves in water, the ions that make up the compound dissociate and disperse throughout the solution. To find the moles of ions present, you first need to know the chemical formula of the compound and the molar mass of its constituent elements. With this information, you can use stoichiometry to determine the mole ratio between the components and calculate the moles of each ion present.
Step 1: Write the balanced chemical equation
The first step is to write the balanced chemical equation for the dissolution process. This shows the relative number of ions present. For example, if sodium chloride (NaCl) dissolves:
NaCl(s) → Na+(aq) + Cl-(aq)
This shows that for every unit of sodium chloride that dissolves, you get one sodium ion and one chloride ion in solution.
Step 2: Determine the molar mass
Use the periodic table to find the molar mass (g/mol) of each element in the compound. For sodium chloride:
- Na: 22.99 g/mol
- Cl: 35.45 g/mol
Add these together to get the molar mass of NaCl: 58.44 g/mol.
Step 3: Find the moles of compound
Use the mass of the compound dissolved (in grams) and its molar mass to calculate the number of moles of compound present:
Moles of compound = Mass (g) / Molar mass (g/mol)
For example, if you dissolve 58.44 g of NaCl in water:
Moles of NaCl = 58.44 g / 58.44 g/mol = 1 mol
Step 4: Relate moles of compound to moles of ions using stoichiometry
The chemical equation shows the mole ratio between the compound and each of its ions. For NaCl dissolving:
NaCl(s) → Na+(aq) + Cl–(aq)
There is a 1:1:1 mole ratio between NaCl, Na+, and Cl–. So if you have 1 mol of NaCl, you will have 1 mol of Na+ and 1 mol of Cl– when it dissociates.
Step 5: Calculate moles of each ion
Using the mole ratio, you can now calculate the moles of each ion present based on the moles of compound.
For the example above with 1 mol of NaCl:
- Moles of Na+: 1 mol NaCl x (1 mol Na+/1 mol NaCl) = 1 mol Na+
- Moles of Cl–: 1 mol NaCl x (1 mol Cl–/1 mol NaCl) = 1 mol Cl–
So dissolving 1 mol (58.44 g) of NaCl gives 1 mol of Na+ ions and 1 mol of Cl– ions.
Example Problem
Let’s do an example problem using these steps:
How many moles of calcium ions and nitrate ions are present when 5.00 g of calcium nitrate, Ca(NO3)2, is dissolved in water?
Step 1: Write the chemical equation
Ca(NO3)2(s) → Ca2+(aq) + 2 NO3–(aq)
Step 2: Determine the molar mass
Ca: 40.08 g/mol
N: 14.01 g/mol
O: 16.00 g/mol
Molar mass of Ca(NO3)2 = 164.10 g/mol
Step 3: Find moles of compound
Moles of Ca(NO3)2 = 5.00 g / 164.10 g/mol = 0.0305 mol
Step 4: Find mole ratio
The equation shows a 1:1:2 mole ratio between Ca(NO3)2, Ca2+, and NO3–
Step 5: Calculate moles of ions
Moles of Ca2+ = 0.0305 mol Ca(NO3)2 x (1 mol Ca2+/1 mol Ca(NO3)2) = 0.0305 mol
Moles of NO3– = 0.0305 mol Ca(NO3)2 x (2 mol NO3–/1 mol Ca(NO3)2) = 0.061 mol
So dissolving 5.00 g of Ca(NO3)2 yields 0.0305 mol of Ca2+ ions and 0.061 mol of NO3– ions.
Conclusion
By following these steps, you can determine the moles of ions present when an ionic compound dissolves:
- Write the balanced chemical equation
- Determine the molar mass of the compound
- Calculate moles of compound from the mass
- Determine the mole ratio from the equation
- Use the mole ratio to calculate moles of each ion
This involves using the chemical formula, stoichiometry, and mole calculations. With some practice, finding the moles of dissolved ions is a straightforward process!
Practice Problems
Try calculating the moles of ions for the following practice problems:
Problem 1
How many moles of aluminum ions and sulfate ions are present when 12.4 g of aluminum sulfate, Al2(SO4)3, is dissolved?
Problem 2
How many moles of calcium ions and hydroxide ions are present when 28.7 g of calcium hydroxide, Ca(OH)2, is dissolved?
Problem 3
How many moles of iron(III) ions and nitrate ions are present when 17.8 g of iron(III) nitrate, Fe(NO3)3, is dissolved?
Tips for Determining Moles of Ions
Here are some useful tips when finding the moles of ions dissolved in water:
- Make sure you have the balanced chemical equation – this gives the mole ratio.
- Use molar mass to convert between mass of compound and moles.
- Pay attention to the subscripts in the formula when calculating molar mass.
- Use stoichiometry and the mole ratio to relate moles of compound to moles of ions.
- Watch out for decimals – make sure you account for significant figures.
- Check your units throughout the calculation.
Common Ionic Compounds
Here are some common ionic compounds you may encounter when calculating moles of dissolved ions:
| Ionic compound | Chemical formula |
|---|---|
| Sodium chloride | NaCl |
| Calcium chloride | CaCl2 |
| Magnesium sulfate | MgSO4 |
| Aluminum nitrate | Al(NO3)3 |
| Lithium carbonate | Li2CO3 |
| Potassium phosphate | K3PO4 |
| Calcium hydroxide | Ca(OH)2 |
| Copper(II) sulfate | CuSO4 |
Be sure you can find the molar mass and write dissociation equations for these common ionic compounds.
Solubility Rules
When dissolving ionic compounds in water, it’s important to consider solubility. Many ionic compounds are soluble and dissociate fully. However, some are only slightly soluble or insoluble in water.
Here are some key solubility rules to remember:
- Most group 1 (alkali) metal salts are soluble.
- Most ammonium salts are soluble.
- Most nitrates, acetates and chlorides are soluble.
- Most sulfate salts are soluble, except calcium, barium, strontium and lead.
- Most hydroxides are insoluble, except group 1 metals and barium.
- Most oxides are insoluble, except group 1 metals and barium.
- Most carbonates, phosphates, sulfides and silicates are insoluble.
When applying these rules, be aware of differences between soluble ionic compounds that fully dissociate (like NaCl) vs. those with limited solubility (like CaCO3).
Limiting Reactant and Percent Yield
When calculating moles of ions, it’s also important to consider limiting reactants and percent yield. If you don’t have enough of a given reactant to reach the expected stoichiometry, the reactant will limit the moles of ions formed.
Also, chemical reactions typically do not convert 100% of reactants into products. You may only get 80% or 90% of the theoretical yield based on stoichiometry. This reduces the actual moles of ions present.
Be sure to account for these factors when calculating moles of ions experimentally rather than just theoretically from stoichiometry.
Using Molarity
Molarity (M) represents the moles of solute dissolved per liter of solution. It can be used as an alternative to mass when calculating moles of ions.
For example, if you need the moles of nitrate ion in a 0.500 L solution of 0.150 M calcium nitrate Ca(NO3)2, you can calculate:
Moles of NO3– = Molarity x Volume
Moles of NO3– = 0.150 mol/L x 0.500 L = 0.075 mol
So molarity provides a convenient way to determine moles of ions based on solution concentration and volume.
Importance of Ionic Strength
The concentration and charge of dissolved ions affects the ionic strength of a solution. Ionic strength influences the behavior of electrolytes and impacts properties like:
- Solubility equilibria
- Reaction rates
- Protein folding/stability
Calculating ion concentrations accurately allows you to control ionic strength and understand its effects. This is critical in fields like biochemistry and environmental chemistry.
Applications
Determining moles of dissolved ions has many real-world applications including:
Water treatment
Calculating ion concentrations allows appropriate water purification and conditioning by methods like ion exchange and reverse osmosis.
Electrochemistry
The moles of ions affects electrochemical cell voltage and current. Accurate ion concentrations optimize battery/fuel cell performance.
Medicine
Proper ion doses in intravenous fluids and medications require knowing the moles of ions present to achieve physiological effects.
Agriculture
Managing nutrient levels for plant growth involves measuring ion moles from fertilizers in soil and irrigation water.
Industry
Ion moles dictate solubility equilibria and reaction rates for processes like ore flotation, catalysis, and microelectronics etching.
So finding moles of dissolved ions has far-reaching benefits across many fields!
Conclusion
In summary, here is the process to determine moles of ions when an ionic compound dissolves in water:
- Write the balanced dissociation equation.
- Find the molar mass of the compound.
- Use the mass dissolved to calculate moles of compound.
- Determine the mole ratio from the equation.
- Calculate moles of each ion using the ratio.
Be sure you understand solubility rules and account for limiting reactants and percent yield. Mastering these stoichiometric calculations is essential for quantifying ion concentrations in solution. With some practice, finding moles of dissolved ions will become second nature!
