In chemistry when the concentration of a solution is accurately known it is called a standard solution. Standard solutions are needed for carrying out chemical experiments in the laboratory. Furthermore, they are needed to get accurate results. Standard solutions are useful in several industries as well.
The concentration of a solution
The amount of moles of a solute in a unit volume of a solution is called the concentration. Simply said, concentration gives an idea about how many moles a chemical is there in a given volume of a solution.
Concentration is measure with the unit moles per cubic decimeter (moldm-3). For this equation number of moles needs to be in “mol” unit and the volume of the solution needs to be in cubic decimeters. (dm3)
Following is the equation for calculating the concentration of a solute in a solution.
Concentration is used to describe a homogenous mixture. A homogenous mixture can be also known as a solution in chemistry.
Practical experiment – Prepare a standard solution
In this practical session let’s see how a standard solution of sodium chloride is made. Here the concentration of the standard solution of sodium chloride is 1 moldm-3 and the volume is 250 cm3
(250 cm3 is equal to 250 ml)
Method of preparing the standard solution
- For this specific solution, we need 14.625 grams of sodium chloride. It needs to be measured accurately from a chemical balance.
- Collect the solute (NaCl) into the watch glass
- Remove the lid of the volumetric flask connect the funnel
- Use the wash bottle to transfer the solute from the watch glass to the volumetric flask via the funnel
- Close the lid and mix the solution properly with around 2/3 of the volumetric flask filled
- Open the lid and fill the volumetric flask with water with the wash bottle
- When reaching the marked level, watch the mark at eye level and add water drop by drop from the wash bottle
- When the mark is reached close the lid and mix the solution again gently and with minimum contact with the bulb of the volumetric flask
- The final step is to label the standard solution. The label should contain the name of the solution, volume and concentration, and the date of preparation.
Equipment required to carry out this experiment
- Volumetric flasks – to accurately measure the volume
- Wash bottle – to wash out the NaCl into the flask and add water dropwise
- Watch glass – to keep the solute (NaCl)
- Chemical balance – to accurately measure the number of grams of the solute
- Funnel – to direct washed-out solution in the volumetric flask
- Sodium chloride – as the solute of the standard solution
Important things to remember
Just like any other chemical experiment in the laboratory accurate results depend on adhering to the correct techniques. The following points need to be kept in mind when handling chemicals and carrying out this experiment in the laboratory.
- Cleanliness of all the equipment used
If the instruments used are not cleaned properly the solution we prepare might be contaminated with other chemicals. So cleaning instruments should be done correctly prior to starting the experiment.
- Weighing the solute accurately
The accurate weight of chemicals is a must for making a standard solution. If the weight of the solute is wrong the final standard solution will be not accurate.
- Washing well and transferring the substance sticking onto the watch glass and the funnel into the flask
If the solute remains outside the volumetric flask that means the stand solution we prepare is going to be inaccurate.
- Mix the solution without touching the bulb of the flask
Touching the bulb of the volumetric flask with the palms of the hand can transfer heat from our body to the solution we are preparing. With some chemicals, this can induce a chemical reaction and the accurate standard solution will not be made due to the transferred heat.
So bulb of the volumetric flask needs to be touched with fingertips and the contact time has to be minimized.
- Use the correct technique for mixing
- Adjusting the final volume carefully
In the final steps, the wash bottle should be used carefully to add water drop by drop. If the volume mark exceeds the entire solution has to be discarded and a new standard solution has to be made from the beginning.
This is because once extra water is added the solution dilutes and if we remove the solution after that, it takes some of its solutes with it resulting in an inaccurate solution.
- Get the required volume by observing the volume mark of the flask at the eye level
- Prevention of the entry of impurities into the solution