If 35.0 mL of 0.210 M HCl reacts with excess Mg, how many mL of hydrogen gas are produced at STP?

Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

mols HCl = M x L = ?

Using the coefficients in the balanced equation, convert mols HCl to mols H2 gas.
Now convert mols H2 gas to L. L H2 = mols H2 x 22.4 L/mol = ? L.

To find out how many milliliters (mL) of hydrogen gas are produced at STP when 35.0 mL of 0.210 M HCl reacts with excess Mg, we need to use stoichiometry.

Step 1: Write down the balanced chemical equation:
Mg(s) + 2HCl(aq) → MgCl2(aq) + H2(g)

Step 2: Use the given information to determine the limiting reactant. Since HCl is given in excess, we can assume that Mg is the limiting reactant.

Step 3: Calculate the number of moles of Mg using the provided volume and molarity of HCl:
35.0 mL HCl * (0.210 moles HCl / 1 L HCl) * (1 L / 1000 mL) = 0.00735 moles HCl

Step 4: Use stoichiometry to find the number of moles of H2 produced. According to the balanced equation, the mole ratio between Mg and H2 is 1:1.
0.00735 moles H2 = 0.00735 moles HCl

Step 5: Convert moles of H2 to volume using the molar volume of gas at STP (22.4 L/mol):
0.00735 moles H2 * (22.4 L H2 / 1 mole H2) = 0.1644 L H2

Step 6: Convert liters to milliliters:
0.1644 L H2 * (1000 mL / 1 L) = 164.4 mL

Therefore, approximately 164.4 mL of hydrogen gas are produced at STP.

To find out how many mL of hydrogen gas are produced at STP, we need to determine the number of moles of hydrogen gas produced and then use the molar volume of a gas at STP to convert moles to volume.

First, let's find the number of moles of hydrogen gas produced. From the balanced equation, we can see that one mole of magnesium (Mg) reacts with two moles of hydrochloric acid (HCl) to produce one mole of hydrogen gas (H2).

Given that we have 35.0 mL of 0.210 M HCl, we can use the concentration and volume to calculate the number of moles of HCl:
moles of HCl = concentration of HCl × volume of HCl in liters

Converting the volume of HCl from mL to liters:
volume of HCl = 35.0 mL × (1 L / 1000 mL) = 0.0350 L

Now we can calculate the number of moles of HCl:
moles of HCl = 0.210 M × 0.0350 L = 0.00735 moles of HCl

Since the mole ratio between HCl and H2 is 2:1, the number of moles of H2 produced will be half of the moles of HCl:
moles of H2 = 0.00735 moles of HCl / 2 = 0.00368 moles of H2

Now, we need to use the molar volume of a gas at STP, which is 22.4 L/mol, to convert the moles of hydrogen gas to volume:
volume of H2 = moles of H2 × molar volume of gas at STP

volume of H2 = 0.00368 moles of H2 × 22.4 L/mol = 0.08243 L = 82.43 mL

Therefore, 82.43 mL of hydrogen gas are produced at STP.