Calculating NaCl Mass For 1 L Of 0.85% Solution

Oct 19, 2025 by ADMIN 48 views

Calculating NaCl Mass for a 1 L, 0.85% Solution

Hey guys! Today, we're diving into a common chemistry problem: figuring out how much sodium chloride ( $NaCl$ ), also known as table salt, you need to weigh out to make a specific solution. This is super practical, especially if you're working in a lab or even just doing some cool science experiments at home. We'll break down the steps to calculate the mass of $NaCl$ needed to prepare 1 L of a 0.85% (m/v) solution, taking into account that the reagent you're using isn't 100% pure (it's 99% pure in this case). Let's get started!

Understanding the Problem

Before we jump into calculations, let's make sure we understand what the question is asking. We need to prepare 1 liter (1 L) of a solution where the concentration of $NaCl$ is 0.85% (m/v). The term “m/v” means “mass/volume,” so 0.85% (m/v) means that there are 0.85 grams of $NaCl$ for every 100 milliliters (mL) of solution. We also need to consider that our $NaCl$ isn't perfectly pure; it's 99% pure, meaning that only 99% of the mass we weigh out will actually be $NaCl$ , and the rest is impurities. Understanding these key concepts is crucial for solving the problem accurately. Think of it like baking – if your ingredients aren't measured correctly, your cake won't turn out right! Similarly, in chemistry, precise measurements are essential for reliable results.

Step 1: Calculate the Required Mass of NaCl (Pure)

First, we need to determine how much pure $NaCl$ we need for our solution. We know that the solution is 0.85% (m/v), which means there are 0.85 grams of $NaCl$ in 100 mL of solution. But we want to make 1 L of solution, which is equal to 1000 mL. So, we need to scale up our calculation. To do this, we can set up a proportion:

\frac{0.85 \text{ g } NaCl}{100 \text{ mL}} = \frac{x \text{ g } NaCl}{1000 \text{ mL}}

Solving for x gives us:

x = \frac{0.85 \text{ g } NaCl \times 1000 \text{ mL}}{100 \text{ mL}} = 8.5 \text{ g } NaCl

So, we need 8.5 grams of pure $NaCl$ to make 1 L of a 0.85% solution. This is a crucial intermediate step, but remember, our $NaCl$ isn't 100% pure, so we're not quite done yet. It’s like knowing how much flour you need for a cake, but then realizing your measuring cup has a small dent, so you need to adjust the amount slightly.

Step 2: Account for the Purity of the Reagent

Now, we need to account for the fact that our $NaCl$ is only 99% pure. This means that for every 100 grams of the reagent we weigh out, only 99 grams are actually $NaCl$ , and 1 gram is impurities. To find out how much of the impure $NaCl$ we need to weigh, we need to adjust our previous result. We can set up another proportion:

\frac{99 \text{ g pure } NaCl}{100 \text{ g impure } NaCl} = \frac{8.5 \text{ g pure } NaCl}{y \text{ g impure } NaCl}

Solving for y gives us:

y = \frac{8.5 \text{ g pure } NaCl \times 100 \text{ g impure } NaCl}{99 \text{ g pure } NaCl} \approx 8.59 \text{ g impure } NaCl

Therefore, we need to weigh out approximately 8.59 grams of the 99% pure $NaCl$ reagent. This adjustment is vital because if we weighed out just 8.5 grams, we wouldn't have enough actual $NaCl$ in our solution, and the concentration would be off. It’s like adding a pinch of salt to your food – too little, and it tastes bland; too much, and it’s overpowering.

Step 3: Final Answer and Practical Considerations

So, the final answer is that you should weigh out approximately 8.59 grams of the 99% pure sodium chloride ( $NaCl$ ) to prepare 1 L of a 0.85% (m/v) solution. But, a few practical tips, guys! When you're in the lab, it's always a good idea to use a high-precision balance to weigh out your chemicals. Balances can have varying degrees of precision, so choosing one appropriate for your needs is essential. Additionally, always use appropriate personal protective equipment (PPE), like gloves and safety glasses, when handling chemicals. Safety first!

Rounding and Significant Figures

In chemistry, it's important to pay attention to significant figures. In our calculation, we started with values like 0.85% and 99%, which have two and two significant figures, respectively. Our final answer should reflect the least precise value we used. In this case, we should round our answer to two significant figures. So, 8.59 grams becomes 8.6 grams. Paying attention to significant figures ensures that our results are reported with the appropriate level of precision, avoiding misleading claims about accuracy. It's like crafting a story – you want to provide enough detail to be clear, but not so much that it becomes convoluted.

Preparing the Solution

Once you've weighed out the $NaCl$ , you'll need to dissolve it in water to make the solution. Here’s a quick guide:

Weigh the NaCl: Carefully weigh out 8.6 grams of the 99% pure $NaCl$ using a balance.
Dissolve in Water: Add the $NaCl$ to a beaker containing less than 1 L of distilled or deionized water (e.g., about 800 mL). Stir until the $NaCl$ is completely dissolved.
Make Up to Volume: Transfer the solution to a 1 L volumetric flask. Add more water until the solution reaches the 1 L mark on the flask. Be sure to check the meniscus (the curve of the liquid) to ensure accurate measurement.
Mix Thoroughly: Stopper the flask and invert it several times to ensure the solution is homogeneous (evenly mixed). Proper mixing is critical for ensuring the concentration is uniform throughout the solution. Think of it like stirring a pot of soup – you want to make sure all the flavors are evenly distributed.

Common Mistakes to Avoid

Let’s chat about some common mistakes people make when tackling problems like this. One frequent error is forgetting to account for the purity of the reagent. As we saw, the difference between 8.5 grams and 8.6 grams might seem small, but it can affect the accuracy of your solution. Another mistake is incorrect unit conversions. Always double-check that you’re converting mL to L (or vice versa) correctly. Finally, carelessness in measurements can lead to inaccuracies. Avoiding these common pitfalls will help you ensure your solutions are accurate and reliable. It’s like proofreading a paper – catching those little errors can make a big difference in the final result.

Why is This Important?

You might be wondering, why bother with all these calculations? Well, accurate solutions are essential in many areas of chemistry, biology, medicine, and even cooking! In labs, precise concentrations are needed for experiments to work correctly. In medicine, intravenous (IV) solutions need to have the right concentration of salts and other substances to be safe for patients. Even in cooking, the right amount of salt can make or break a dish. Understanding solution preparation is a fundamental skill that has widespread applications, so mastering these calculations is a solid investment in your scientific toolkit. It's like learning the basics of a language – once you have the grammar and vocabulary down, you can express yourself clearly and effectively.

Practice Problems

To really nail this concept, here are a few practice problems you can try:

How much glucose ( $C_6H_{12}O_6$ ) is needed to prepare 500 mL of a 5% (m/v) solution, assuming the glucose is 100% pure?
You need to make 2 L of a 0.9% (m/v) saline solution ( $NaCl$ ). If your $NaCl$ is 98% pure, how much should you weigh out?
What mass of potassium chloride ( $KCl$ ) is required to prepare 250 mL of a 1.5% (m/v) solution, given that the $KCl$ has a purity of 95%?

Work through these problems, and you’ll become a solution-making pro in no time! Practice makes perfect, and the more you work with these calculations, the more confident you'll become. It's like learning a musical instrument – the more you practice, the more fluent you become.

Conclusion

So, there you have it! We’ve walked through the steps to calculate the mass of $NaCl$ needed to prepare a specific solution, taking into account reagent purity. Remember to (1) calculate the mass of pure $NaCl$ required, (2) adjust for the purity of the reagent, and (3) consider practical aspects like significant figures and accurate measurement techniques. Mastering these calculations is a key skill in chemistry and beyond. Keep practicing, stay curious, and happy solution-making, guys! You've got this! Just like any skill, mastering solution preparation takes time and effort, but with a solid understanding of the fundamentals and a bit of practice, you'll be creating solutions like a seasoned pro. So keep exploring, keep learning, and never stop asking questions. The world of chemistry is full of fascinating puzzles just waiting to be solved!