Chemistry Unit 9: Lesson 1 Solutions and Molarity

Solutions

Like we mentioned before a majority of chemistry involves working with aqueous mixtures or solutions.

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Even when we say "chemicals" most of the time​ we picture in our mind a liquid of some sort.

Solutions

Solutions are defined as mixture of 2 or more substances in a single phase. Most commonly we think of mixtures in the liquid phase.

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A solution has two distinct parts that make it up and also must be defined. ​

The Solvent and the Solute​.

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Solutions

​The main ingredient or the part of the solution that does the dissolving is known as the solvent.

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The other ingredient(s) or the part(s) of the ​solution that is/are dissolved are known as the solutes.

Solutions

But how do we describe or measure how strong a solution is?

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Think of it like making a glass of sweet tea, what is a word used for a glass that has a lot more sugar than another?​

Concentration​

Concentration is defined as a specific measurement of the amount of solute in relation to the solvent.

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There are multiple ways in which concentration is expressed. Commonly we see everyday products measured in percent.

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For example Vinegar is 5% acetic acid. ​

Concentration​

In chemistry this can be used, but more often than not we utilize Molarity.

Molarity is used because it can be very easily plugged into balanced equations and used to solve stoichiometric ratios

Concentration​

Using our equation a few items we must be careful with. First or volume of solution must be in L. It is common for it to be measured in mL. To convert we would use the stair steps or know that 1 L = 1000 mL. So if we started in mL we would divide by 1000 or move the decimal to the left 3 places to convert to L.

Concentration​

Also we commonly measure our solutes on a scale before dissolving them into our solvent to form a solution. How do we convert grams to moles?​

Concentration​

Also we commonly measure our solutes on a scale before dissolving them into our solvent to form a solution. How do we convert grams to moles?​

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We divide by molar​ mass. If we have a solution we can also determine the mass of solute that must already be dissolved in it if we have the concentration by finding moles and multiplying by molar mass.

Tips

One tip when creating a solution in a laboratory is to make sure and add the solute to the final container first. The volume is the final volume the solution should be. Even though the volume the solute will take up is very small it is still meaningful. You would simply add solvent till the volume reached the calculated volume for the calculated Molarity. If you measure separately and add together most often your concentration will be lower than expected because your volume is higher​ than calculated.

Example

What would be the molarity of a glass of salt water if 14 g of NaCl is dissolved to form a solution that has a volume of 500 mL? The molar mass of NaCl is 58.44 g/mol.

Example

What would be the molarity of a glass of salt water if 14 g of NaCl is dissolved to form a solution that has a volume of 500 mL? The molar mass of NaCl is 58.44 g/mol.

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14 g / 58.44 = 0.24 moles 0.24 moles / 0.5 L = 0.48 M NaCl​

Example

What volume must 108 grams of NaCl be watered down to so that the molarity of the solution is 0.80 M? The molar mass of NaCl is 58.44 g/mol.

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Example

What volume must 108 grams of NaCl be watered down to so that the molarity of the solution is 0.80 M? The molar mass of NaCl is 58.44 g/mol.

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108​/ 58.44 = 1.85 moles 0.8 = 1.85 / x x = 2.31 L

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